How PHP executes on the server - Mechanics & Internals

Overview - How PHP executes on the server

What is it?

PHP is a programming language that runs on a web server to create web pages dynamically. When a user requests a PHP page, the server reads the PHP code, runs it, and sends the result as a web page to the user's browser. This process happens behind the scenes, so users only see the final web page, not the PHP code itself.

Why it matters

Without PHP running on the server, websites would be static and unable to change content based on user actions or data. PHP allows websites to be interactive, like showing personalized messages or handling form submissions. This makes the web more useful and engaging for everyone.

Where it fits

Before learning how PHP executes, you should understand basic web concepts like how browsers and servers communicate using HTTP. After this, you can learn about PHP syntax, how to write PHP scripts, and how PHP interacts with databases to build full web applications.

Mental Model

Core Idea

PHP code runs on the server to create web pages dynamically, sending only the final result to the user's browser.

Think of it like...

Imagine a chef (the server) who follows a recipe (PHP code) to prepare a dish (web page). The customer (browser) only sees the finished dish, not the recipe or cooking process.

User Browser
   ↓ HTTP Request
┌─────────────────────┐
│     Web Server      │
│  ┌───────────────┐  │
│  │ PHP Interpreter│  │
│  └───────────────┘  │
│       Executes       │
│      PHP Code        │
│       Generates      │
│     HTML Output      │
└─────────────────────┘
   ↑ HTTP Response
User Browser receives final HTML page

Build-Up - 6 Steps

1

Foundation

What Happens When You Visit a PHP Page

Concept: Understanding the basic request-response cycle involving PHP.

When you type a URL that ends with .php in your browser, the browser sends a request to the web server. The server sees the .php file and knows it needs to run the PHP code inside it. The server runs the PHP code and sends back the result as a web page.

Result

The browser shows the web page created by the PHP code.

Knowing that PHP runs on the server and not in the browser helps you understand why users never see PHP code directly.

2

Foundation

Role of the PHP Interpreter

Concept: Introducing the PHP interpreter as the program that runs PHP code on the server.

The PHP interpreter is software installed on the server that reads PHP code and executes it line by line. It converts PHP instructions into actions like fetching data or creating HTML. Without the interpreter, the server would not understand PHP files.

Result

PHP code is transformed into HTML or other outputs the browser can display.

Understanding the interpreter clarifies how PHP code turns into something the browser can use.

3

Intermediate

PHP Execution Flow on the Server

🤔Before reading on: Do you think PHP code runs all at once or line by line? Commit to your answer.

Concept: How PHP processes code step-by-step during a request.

When the server receives a PHP request, the interpreter starts at the top of the PHP file and runs each line in order. It can include other files, run functions, and interact with databases. After finishing, it sends the combined output as the response.

Result

The final output is a complete web page ready for the browser.

Knowing PHP runs line by line helps you debug and organize your code logically.

4

Intermediate

How PHP Handles Dynamic Content

🤔Before reading on: Does PHP generate the same page every time or can it change? Commit to your answer.

Concept: PHP can create different pages based on data or user input.

PHP can use information like form inputs, cookies, or database data to change what it sends back. For example, it can show a welcome message with your name or list products from a database. This makes websites interactive and personalized.

Result

Users see web pages that change depending on their actions or data.

Understanding dynamic content shows why PHP is powerful for real-world websites.

5

Advanced

PHP and Web Server Integration

🤔Before reading on: Do you think PHP runs independently or works closely with the web server? Commit to your answer.

Concept: How PHP connects with web servers like Apache or Nginx to handle requests.

PHP usually runs as a module or separate process alongside the web server. When a PHP page is requested, the server passes the request to PHP, waits for the output, then sends it to the browser. This cooperation ensures efficient handling of many requests.

Result

Smooth delivery of dynamic web pages to many users at once.

Knowing this integration helps you understand server setup and performance tuning.

6

Expert

PHP Opcode Caching and Performance

🤔Before reading on: Do you think PHP code is re-interpreted every time or can it be optimized? Commit to your answer.

Concept: How PHP uses caching to speed up repeated code execution.

PHP converts code into an intermediate form called opcode before running it. Opcode caching stores this form in memory so PHP doesn't re-parse the code on every request. This reduces server load and speeds up page delivery.

Result

Faster response times and better server efficiency for PHP websites.

Understanding opcode caching reveals how PHP scales to handle high traffic.

Under the Hood

When a PHP file is requested, the web server forwards it to the PHP interpreter. The interpreter parses the PHP code into tokens, compiles these into opcode (a low-level set of instructions), and executes them. During execution, PHP can interact with databases, file systems, and other services. The output, usually HTML, is sent back to the server, which then sends it to the user's browser. This entire process happens for each request, but opcode caching can store compiled code to speed up future requests.

Why designed this way?

PHP was designed to be easy to embed in HTML and run on servers to create dynamic pages quickly. The interpreter model allows flexibility and simplicity, letting developers write code that runs immediately without a separate compile step. Opcode caching was added later to improve performance without changing the developer experience. Alternatives like compiled languages exist but are less flexible for rapid web development.

┌───────────────┐      ┌─────────────────────┐      ┌───────────────┐
│ User Browser  │─────▶│   Web Server (e.g.,  │─────▶│ PHP Interpreter│
│ (Sends HTTP   │      │ Apache, Nginx)       │      │ (Parses &     │
│  Request)     │      │                     │      │  Executes     │
└───────────────┘      └─────────────────────┘      └───────────────┘
                                                      │
                                                      ▼
                                               ┌───────────────┐
                                               │  Database or  │
                                               │  Other APIs   │
                                               └───────────────┘
                                                      │
                                                      ▼
                                               ┌───────────────┐
                                               │  Output (HTML)│
                                               └───────────────┘
                                                      │
                                                      ▼
┌───────────────┐      ┌─────────────────────┐      ┌───────────────┐
│ User Browser  │◀─────│   Web Server         │◀─────│ PHP Interpreter│
│ (Receives    │      │                     │      │ (Sends Output) │
│  Response)   │      │                     │      │               │
└───────────────┘      └─────────────────────┘      └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does PHP code run in the user's browser? Commit to yes or no.

Common Belief: PHP code runs on the user's browser just like JavaScript.

Reality: PHP runs only on the server; the browser receives only the output (usually HTML).

Why it matters: Thinking PHP runs in the browser leads to confusion about security and how dynamic pages work.

Quick: Is PHP code compiled once and then runs forever? Commit to yes or no.

Common Belief: PHP code is compiled once and then runs repeatedly without re-parsing.

Reality: PHP code is parsed and executed on every request, though opcode caching can speed this up.

Why it matters: Assuming PHP is pre-compiled can cause misunderstandings about performance and deployment.

Quick: Does the web server itself execute PHP code? Commit to yes or no.

Common Belief: The web server directly runs PHP code without any helper.

Reality: The web server passes PHP files to the PHP interpreter, which runs the code.

Why it matters: Misunderstanding this can lead to incorrect server setup and troubleshooting.

Quick: Does PHP always generate the same output for every user? Commit to yes or no.

Common Belief: PHP pages are static and do not change per user.

Reality: PHP can generate different content based on user input, sessions, or database data.

Why it matters: Ignoring PHP's dynamic nature limits understanding of interactive web applications.

Expert Zone

1

PHP's opcode caching is transparent to developers but can cause confusing bugs if cached code doesn't update after changes.

2

PHP can run in different modes (module, CGI, FastCGI), affecting performance and how it integrates with the web server.

3

The PHP interpreter uses a stack-based virtual machine internally to execute opcode, which influences how functions and variables behave.

When NOT to use

PHP is less suitable for long-running background tasks or real-time applications; alternatives like Node.js or Go are better for those. Also, for very high-performance needs, compiled languages or specialized frameworks might be preferred.

Production Patterns

In production, PHP is often paired with opcode caches like OPcache, run behind web servers using FastCGI for speed, and combined with databases like MySQL. Frameworks like Laravel structure PHP code for maintainability, and deployment uses tools to manage code versions and caching.

Connections

Client-Server Model

PHP execution is a specific example of the client-server communication pattern.

Understanding PHP's role clarifies how servers process requests and send responses in many networked applications.

Compiler and Interpreter Theory

PHP uses an interpreter that compiles code to opcode before execution, blending both concepts.

Knowing this helps understand performance trade-offs and how languages execute code.

Assembly Language Execution

PHP opcode is like assembly instructions for a virtual machine inside the interpreter.

This connection reveals how high-level code translates into low-level operations, similar to how CPUs run machine code.

Common Pitfalls

#1 Expecting PHP code to run in the browser and trying to debug it there.

Wrong approach:

Correct approach:

Run PHP code on the server and view output in browser as HTML, not in browser console.

Root cause:

Misunderstanding that PHP is server-side, not client-side.

#2 Editing PHP files but not restarting or clearing opcode cache, so changes don't appear.

Wrong approach:

Change PHP code but see old output because OPcache is still serving cached code.

Correct approach:

Clear or disable OPcache after code changes during development.

Root cause:

Not knowing about opcode caching and its effect on code updates.

#3 Configuring web server to serve PHP files as plain text instead of executing them.

Wrong approach:

Accessing PHP page and seeing raw PHP code in browser.

Correct approach:

Configure web server to pass PHP files to PHP interpreter for execution.

Root cause:

Incorrect server setup or missing PHP module.

Key Takeaways

PHP runs on the server, not in the browser, turning code into web pages dynamically.

The PHP interpreter reads and executes PHP code line by line for each web request.

PHP works closely with web servers to handle requests and deliver responses efficiently.

Opcode caching improves PHP performance by storing compiled code between requests.

Understanding PHP's execution model helps avoid common mistakes and optimize web applications.