How PHP Works on the Server

Part of the course: php for beginners

1. Introduction to PHP

PHP (Hypertext Preprocessor) is a popular open-source scripting language primarily used for web development. It is designed to run on the server side, meaning that PHP code is executed on the web server before the resulting HTML is sent to the client’s browser. This allows developers to create dynamic and interactive web pages that can respond to user input, access databases, and generate customized content in real time.

PHP is embedded within HTML, making it easy to integrate with existing web pages. It works seamlessly with many web servers such as Apache and Nginx and supports various databases, including MySQL, PostgreSQL, and SQLite. Because of its simplicity, flexibility, and extensive community support, PHP remains one of the most widely used languages for building websites and web applications.

2. What Happens When You Request a PHP Page

When a user requests a PHP page by entering a URL in their browser or clicking a link, a sequence of server-side processes begins behind the scenes. Unlike static HTML files, which are sent directly to the browser, PHP pages must first be processed by the web server and PHP interpreter before the final HTML output is delivered to the user.

Here’s what happens step by step:

  1. Browser Sends a Request:
    The client (browser) sends an HTTP request to the web server, asking for a specific PHP file (for example, index.php).

  2. Web Server Receives the Request:
    The web server (such as Apache or Nginx) checks the file type. When it sees a .php file, it knows the request should be handled by the PHP interpreter instead of being sent directly.

  3. Server Passes the Request to PHP:
    The server forwards the PHP file to the PHP engine (via modules like mod_php or PHP-FPM). The PHP engine reads the code and executes it line by line on the server.

  4. PHP Executes the Code:
    During execution, PHP can interact with databases, read or write files, perform calculations, or process form data.

  5. PHP Generates Output:
    The PHP interpreter produces an output—usually HTML, but it can also generate JSON, XML, or other formats.

  6. Server Sends the Response Back:
    The resulting HTML (or other output) is sent back to the client’s browser through the web server.

  7. Browser Displays the Page:
    The browser receives the response and renders the content on the screen for the user to see.

In short, PHP acts as a bridge between the server and the client, dynamically generating web pages based on user requests, logic, and data from databases or other sources.

3. The Role of the Web Server (Apache, Nginx, etc.)

The web server plays a crucial role in handling PHP requests. It acts as the middle layer between the client (browser) and the server-side PHP interpreter. Its main job is to receive HTTP requests from clients, determine how to handle them, and send back the correct response—often in the form of an HTML page.

Here’s how the web server works in the context of PHP:

  1. Receiving the Request:
    When a user requests a PHP page, the web server (such as Apache or Nginx) receives the incoming HTTP request and identifies the target resource, such as index.php.

  2. Routing the Request:
    The server checks its configuration to decide how to process the file.

    • For static files (like .html, .css, or .jpg), it serves them directly.

    • For PHP files, it passes the request to the PHP handler.

  3. Passing Control to PHP:
    The web server uses a communication method (like mod_php, FastCGI, or PHP-FPM) to send the PHP file to the PHP interpreter for processing. The interpreter executes the PHP code on the server and generates the resulting output (usually HTML).

  4. Receiving Output from PHP:
    Once the PHP interpreter finishes execution, it sends the generated output back to the web server.

  5. Sending the Response to the Client:
    Finally, the web server sends the response (typically an HTML page) to the client’s browser, completing the request–response cycle.

Different web servers handle this process slightly differently:

  • Apache often uses the mod_php module to run PHP directly within the server process.

  • Nginx, on the other hand, communicates with PHP through FastCGI or PHP-FPM, which separates the web server from the PHP engine for better performance and scalability.

In short, the web server’s role is to manage requests, delegate PHP processing, and deliver the final content back to users efficiently.

5. PHP Execution Process Step-by-Step

When a PHP file is requested and handed to the PHP interpreter, the code goes through several internal stages before producing the final output. These stages are handled primarily by the Zend Engine, which is the core of PHP’s runtime system. Understanding these steps helps explain how PHP converts human-readable code into machine-executable instructions.

1. Parsing the PHP Script

At this stage, the PHP interpreter reads the source code and checks its syntax.
The script is broken down into smaller elements such as variables, functions, operators, and control structures. If there are any syntax errors (for example, missing semicolons or unclosed brackets), PHP will stop execution and return an error message.

2. Tokenizing and Compilation

Once the script passes syntax validation, PHP begins tokenizing—it converts the raw text into tokens, which are small units representing the different components of the code (like keywords, variables, and operators).
Then, the PHP compiler takes these tokens and translates them into an intermediate representation called opcodes (operation codes). These opcodes are low-level instructions that the Zend Engine can understand and execute efficiently.

3. Opcode Generation

During this phase, the compiler produces a set of opcodes that represent the logic of the PHP script.
For example, an opcode might tell the engine to add two numbers, call a function, or output text.
This intermediate form allows PHP to execute the code much faster than interpreting each line repeatedly.

(In some cases, these opcodes are stored in memory or cached by systems like OPcache, allowing future requests to skip parsing and compilation, which greatly improves performance.)

4. Execution by the Zend Engine

Finally, the Zend Engine executes the generated opcodes step by step.
During execution, it performs all the necessary operations—such as arithmetic calculations, function calls, database interactions, or file operations—and produces the final output.
This output (usually HTML) is then sent back to the web server, which delivers it to the user’s browser.


The PHP execution process can be thought of as a pipeline —
Code → Parsing → Tokenizing → Compilation → Opcode Generation → Execution → Output.
This structured process ensures PHP code runs efficiently and securely on the server before being displayed to the end user.

6. Interaction with Databases and External Resources

One of PHP’s most powerful capabilities is its ability to communicate with databases and external systems. This is what allows websites and web applications to display dynamic content, handle user data, and perform complex server-side operations.

1. Connecting to Databases

PHP can connect to many types of databases, such as MySQL, PostgreSQL, SQLite, and MariaDB.
This connection is typically established using built-in PHP extensions like:

  • MySQLi (MySQL Improved) – Provides both procedural and object-oriented interfaces.

  • PDO (PHP Data Objects) – Offers a flexible, database-independent interface that supports many database systems.

Through these connections, PHP can:

  • Execute SQL queries (e.g., SELECT, INSERT, UPDATE, DELETE).

  • Retrieve and display data from database tables.

  • Validate user input before saving it.

  • Manage transactions and handle database errors securely.

Example:
When a user logs into a website, PHP retrieves their data from the database, checks the credentials, and returns a personalized response.

2. Working with External Resources

Beyond databases, PHP can interact with various external resources such as:

  • APIs (Application Programming Interfaces): PHP can send or receive data over HTTP using functions like file_get_contents() or cURL. This allows integration with third-party services (e.g., payment gateways, social media APIs, weather services).

  • Files and Directories: PHP can read, write, upload, or modify files on the server, which is essential for content management systems or file upload features.

  • Email Servers: PHP scripts can send emails using built-in functions like mail() or through external libraries (e.g., PHPMailer).

3. Security and Validation

When interacting with databases or external systems, security is crucial. PHP developers must prevent issues such as:

  • SQL Injection (by using prepared statements).

  • Cross-Site Scripting (XSS) when displaying data.

  • Unauthorized access to APIs or files.

By following best practices, PHP ensures that data exchanges remain safe and reliable.

In summary:
PHP serves as the bridge between your web application and the data it relies on. Whether pulling information from a database or sending data to an API, PHP handles these interactions securely and efficiently to create dynamic, data-driven websites.

7. Output Buffering and Sending Data to the Browser

Once the PHP script has been executed and the output is ready, it must be sent back to the client’s browser through the web server. However, before that happens, PHP uses a mechanism called output buffering to control when and how data is sent.

1. What Is Output Buffering?

Normally, PHP sends output (like HTML, text, or JSON) to the browser immediately as it is generated.
However, with output buffering, PHP temporarily stores this output in memory — in a buffer — before sending it.

This approach offers several advantages:

  • Developers can modify or manipulate the output before it’s sent.

  • It prevents “headers already sent” errors, since HTTP headers can still be changed while buffering.

  • It can improve performance by reducing the number of times data is transmitted to the browser.

Output buffering can be started manually using the function ob_start(), and the content can be sent (or flushed) with ob_end_flush() or flush().

2. How PHP Sends Data to the Browser

After all processing is complete — and once output buffering (if used) is finished — PHP passes the final output (usually HTML) to the web server.
The web server then wraps this content inside an HTTP response and sends it over the internet to the client’s browser.

The response typically includes:

  • HTTP Headers: Information about content type (e.g., text/html), encoding, caching, or cookies.

  • Response Body: The actual page content generated by the PHP script.

3. Browser Rendering

When the browser receives the response:

  • It reads the headers to understand how to handle the data.

  • It then parses and renders the HTML (and any linked CSS or JavaScript) on the screen.

From the user’s perspective, this all happens almost instantly — but behind the scenes, PHP and the web server have carefully managed how and when that data is delivered.

In summary:
Output buffering gives PHP developers more control over when data leaves the server. It ensures that the entire response — including headers and content — is properly prepared before being sent to the browser, resulting in smoother, more reliable page delivery.

8. Error Handling and Logging

Error handling and logging are essential parts of PHP’s execution process. They help developers detect, understand, and fix problems that occur during script execution, ensuring that websites and applications run smoothly and securely.

1. Understanding PHP Errors

In PHP, errors can occur for many reasons — such as syntax mistakes, missing files, database connection failures, or incorrect function calls. PHP categorizes these issues into different levels, including:

  • Parse errors – Occur when the PHP code has invalid syntax.

  • Fatal errors – Stop the script completely (e.g., calling an undefined function).

  • Warnings – Indicate a problem but allow the script to continue.

  • Notices – Inform about non-critical issues, such as using an undefined variable.

Properly handling these errors prevents unexpected behavior and improves user experience.

2. Error Handling Techniques

PHP provides several methods to handle errors gracefully:

  • try...catch blocks: Used to handle exceptions without breaking the entire script.

    try {
    // Code that may cause an error
    $result = divide(10, 0);
    } catch (Exception $e) {
    echo "Error: " . $e->getMessage();
    }

  • Custom error handlers: Developers can define their own error-handling functions using set_error_handler().

  • Error reporting levels: The error_reporting() function allows control over which types of errors are displayed or ignored.

By using these tools, developers can prevent users from seeing raw error messages, which might expose sensitive information.

3. Logging Errors

Instead of showing errors to users, PHP can log them to files for developers to review later. This is handled by:

  • The error_log() function, which writes messages to a specified log file.

  • The php.ini configuration file, where you can set:

    log_errors = On
    error_log = /var/log/php_errors.log

Error logs are crucial for diagnosing issues in production environments without interrupting the user experience.

4. Display vs. Log

During development, it’s common to display errors directly on the screen for debugging.
In production, however, displaying errors is risky — instead, they should be logged silently to protect security and professionalism.

In summary:
PHP’s error handling and logging mechanisms ensure that problems are detected, recorded, and managed effectively. Good error handling prevents crashes, maintains security, and makes it easier for developers to debug and maintain PHP applications.

9. PHP and Server-Side Security Considerations

Security is one of the most important aspects of server-side programming, and PHP provides many tools and best practices to help developers protect their applications from common web threats. Since PHP runs on the server, it often deals with sensitive data — such as user credentials, database information, and form inputs — making secure coding essential.

1. Input Validation and Sanitization

User input is one of the main sources of vulnerabilities. Attackers can exploit unvalidated input to inject malicious data into your system.
To prevent this:

  • Always validate user input to ensure it matches the expected type, format, or value range.

  • Sanitize data before using it in queries or displaying it on the page (e.g., using htmlspecialchars() to prevent HTML injection).

Example:

$username = htmlspecialchars($_POST['username'], ENT_QUOTES, 'UTF-8');

2. Preventing SQL Injection

SQL Injection occurs when malicious users manipulate SQL queries through form inputs or URLs.
To prevent this:

  • Use prepared statements with PDO or MySQLi instead of directly embedding variables in SQL.

    $stmt = $pdo->prepare("SELECT * FROM users WHERE email = ?");
    $stmt->execute([$email]);

This ensures that user input is treated as data, not executable SQL code.

3. Cross-Site Scripting (XSS) Protection

XSS attacks happen when attackers inject JavaScript or HTML code into web pages viewed by others.
To prevent XSS:

  • Escape all dynamic content before outputting it to the browser.

  • Use built-in functions like htmlspecialchars() or strip_tags().

  • Implement Content Security Policy (CSP) headers for additional protection.

4. Secure File Handling

If your application allows users to upload files, it’s crucial to verify:

  • The file type (e.g., allow only .jpg, .png, .pdf).

  • The file size (to prevent denial-of-service attacks).

  • The upload directory (store files outside the web root to avoid direct access).

Always rename uploaded files and avoid executing them on the server.

5. Session and Cookie Security

PHP uses sessions and cookies to track user authentication and preferences.
To secure them:

  • Use HTTPS to encrypt session data.

  • Set secure session options in php.ini, such as:

    session.cookie_httponly = 1
    session.cookie_secure = 1

  • Regenerate session IDs after login to prevent session hijacking.

6. Error Message Management

Detailed error messages can reveal sensitive information about the server or code structure.
In production:

  • Disable display of errors (display_errors = Off).

  • Log them privately (log_errors = On).

7. Keeping PHP and Dependencies Updated

Always keep your PHP version, frameworks, and libraries up to date. Security patches are regularly released to fix vulnerabilities. Using outdated versions exposes your server to known exploits.

In summary:
PHP security is about controlling user input, handling data safely, managing sessions securely, and keeping your software updated. Following these practices helps protect both your application and your users from the most common server-side threats.

10. Caching and Performance Optimization

Caching and performance optimization are crucial for making PHP applications run faster, handle more users, and reduce server load. Since PHP generates dynamic content by executing code and often accessing databases, optimizing performance ensures that users experience quick and responsive websites.

1. What Is Caching?

Caching is the process of storing frequently accessed data or generated results temporarily, so that future requests can be served more quickly — without repeating the same processing steps.
In PHP, caching can occur at several levels:

  • Opcode caching

  • Data caching

  • Page or output caching

2. Opcode Caching

When PHP executes a script, it normally parses and compiles the code into opcodes (low-level instructions) before running them. This process happens every time a page is loaded.
With Opcode caching (such as OPcache, built into PHP), these compiled opcodes are stored in memory.
On subsequent requests, PHP can skip parsing and compilation, directly executing the cached version — significantly improving performance.

Example configuration in php.ini:

opcache.enable = 1
opcache.memory_consumption = 128
opcache.max_accelerated_files = 10000

3. Data Caching

Many PHP applications (especially those using databases) repeatedly fetch the same data. Instead of querying the database every time, data can be stored in memory using caching systems like:

  • Redis

  • Memcached

By caching query results, session data, or configuration settings, the application can respond much faster and reduce database load.

4. Page and Output Caching

If a page’s content doesn’t change frequently, PHP can store the entire output (HTML) and serve it directly to users without re-running the script.
This is often used in:

  • Content Management Systems (CMS) like WordPress.

  • Frameworks that support view caching (e.g., Laravel’s Blade cache).

You can enable simple output caching in PHP using:

ob_start();
include('page.php');
file_put_contents('cache.html', ob_get_contents());
ob_end_flush();

5. Code and Query Optimization

Besides caching, PHP performance depends on efficient code and database design. Some best practices include:

  • Minimizing loops and unnecessary function calls.

  • Using efficient database queries and proper indexing.

  • Loading only required files using require_once() or autoloaders.

  • Compressing and minimizing client-side assets (HTML, CSS, JS).

6. Server and Configuration Tuning

Performance can also be improved at the server level by:

  • Using PHP-FPM for faster request handling.

  • Configuring a reverse proxy like Nginx or Varnish for caching and load balancing.

  • Using Content Delivery Networks (CDNs) to serve static assets closer to users.

In summary:
Caching and optimization transform PHP from a simple scripting environment into a high-performance platform. By reducing redundant computations and optimizing code, databases, and servers, PHP applications can serve more users efficiently and deliver a smoother user experience.

11. The Lifecycle of a PHP Request

Understanding the lifecycle of a PHP request helps explain how PHP processes a user’s request from start to finish. This lifecycle involves multiple layers — from the web server receiving the request to the PHP interpreter generating output and sending it back to the browser.

1. Client Sends a Request

The lifecycle begins when a user interacts with a website — for example, by entering a URL, clicking a link, or submitting a form. The browser sends an HTTP request to the web server hosting the PHP application.

2. Web Server Receives the Request

The web server (Apache, Nginx, or others) receives the request and examines the requested file type:

  • Static content (HTML, CSS, JS, images) is served directly.

  • Dynamic content (PHP files) is forwarded to the PHP interpreter for processing.

3. PHP Interpreter Processes the Request

The PHP interpreter handles the dynamic file through a series of internal steps:

  1. Parsing: PHP checks the script for syntax errors.

  2. Tokenizing and Compilation: The script is converted into tokens and then compiled into opcodes.

  3. Execution: The Zend Engine executes the opcodes, performing operations such as calculations, file I/O, and database queries.

4. Interaction with Databases and External Systems

During execution, PHP may interact with databases, APIs, or other external resources to gather or process information needed for the response.

5. Output Generation

PHP generates output — typically HTML, JSON, or XML — which may be temporarily held in output buffers. This allows PHP to modify, optimize, or manipulate the content before sending it.

6. Sending Response to the Web Server

Once the output is ready, PHP passes it back to the web server. The server attaches HTTP headers (content type, cookies, caching information) and prepares the full HTTP response.

7. Web Server Sends Response to Client

Finally, the web server sends the response over the internet to the client’s browser. The browser parses the HTML, loads additional resources (CSS, JS, images), and renders the final page for the user.

8. Cleanup

After sending the response, PHP automatically cleans up:

  • Closes database connections.

  • Frees memory allocated for variables and scripts.

  • Clears output buffers if not already flushed.

This ensures that each request starts fresh and does not interfere with subsequent requests.

In summary:
The lifecycle of a PHP request is a well-coordinated process where the web server, PHP interpreter, and external resources work together to transform a user’s request into a fully rendered page. Each step — from request reception to cleanup — is essential for performance, security, and reliability.

12. Summary and Key Takeaways

PHP is a powerful server-side scripting language that enables developers to create dynamic, data-driven websites and applications. Understanding how PHP works on the server helps explain why it remains widely used and how it interacts with web servers, databases, and client browsers.

Key Points to Remember:

  1. Server-Side Execution: PHP scripts are processed on the server before any output is sent to the browser, allowing dynamic content generation.

  2. Web Server Role: Servers like Apache or Nginx receive requests, route PHP files to the interpreter, and return the final output to clients.

  3. PHP Interpreter Workflow: PHP parses code, generates opcodes, and executes them using the Zend Engine, optionally interacting with databases and external APIs.

  4. Output Handling: Output buffering allows PHP to control when content is sent to the browser, ensuring headers and content are properly managed.

  5. Error Handling and Security: Proper error handling, input validation, and security practices prevent vulnerabilities such as SQL injection, XSS, and session hijacking.

  6. Performance Optimization: Caching (opcode, data, and output) and code optimization reduce server load and improve response times.

  7. Request Lifecycle: Each PHP request follows a structured lifecycle — from receiving the HTTP request, processing the script, generating output, to cleaning up resources.

  8. Modern Server Environments: Using technologies like PHP-FPM and reverse proxies further enhances performance, scalability, and reliability.

Final Thought:
By understanding the server-side workings of PHP, developers can write more efficient, secure, and maintainable applications. PHP’s combination of flexibility, simplicity, and robust server integration makes it an enduring choice for modern web development.

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