How PHP executes on the server - Visual Walkthrough

Concept Flow - How PHP executes on the server

Client sends HTTP request

↓

Web Server receives request

↓

Web Server detects PHP file

↓

PHP Engine starts processing

↓

PHP code is parsed and executed

↓

Output (HTML/JSON/etc) generated

↓

Web Server sends response back to client

↓

Client receives response

This flow shows how a PHP file is requested by a client, processed by the server, executed by PHP, and the result sent back.

Execution Sample

PHP

<?php
echo "Hello, world!";
?>

This PHP code outputs the text 'Hello, world!' when executed on the server.

Execution Table

Step	Action	Details	Result
1	Client sends HTTP request	Request for PHP file (e.g., index.php)	Request received by web server
2	Web server detects PHP file	Recognizes .php extension	Passes request to PHP engine
3	PHP engine starts processing	Reads PHP code from file	Code ready to execute
4	PHP code parsed and executed	Executes echo statement	Generates output: Hello, world!
5	Output sent to web server	Output captured as response body	Response ready to send
6	Web server sends response	Sends HTTP response with output	Client receives 'Hello, world!'
7	Execution ends	No more code to run	Process complete

💡 Execution stops after PHP code finishes and response is sent back to client

Variable Tracker

Variable	Start	After Step 4	Final
Output buffer	""	"Hello, world!"	"Hello, world!"

Key Moments - 3 Insights

Why does the web server pass the request to PHP engine?

Because the requested file has a .php extension, the web server knows it must run the PHP engine to process the code before sending output (see execution_table step 2).

Does the client see the PHP code?

No, the client only receives the output generated by PHP, not the PHP code itself (see execution_table step 6).

What happens if PHP code has errors?

PHP engine stops execution and usually sends an error message back to the web server, which then sends it to the client instead of normal output.

Visual Quiz - 3 Questions

Look at the execution table, at which step does PHP start running the code?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Check the 'Action' column where PHP code is parsed and executed (step 4).

At which step does the client receive the output?

A Step 4

B Step 6

C Step 2

D Step 7

💡 Hint

Look for when the web server sends the response back to the client (step 6).

If the requested file was not a PHP file, what would change in the flow?

A PHP engine would still execute code

B Client would receive PHP code as output

C Web server would not pass request to PHP engine

D Execution would stop at step 4

💡 Hint

Refer to step 2 where the web server detects the file type.

Concept Snapshot

PHP execution on server:
1. Client sends HTTP request for .php file
2. Web server detects PHP and passes request to PHP engine
3. PHP engine parses and runs PHP code
4. Output generated (HTML, JSON, etc.)
5. Web server sends output back to client
Key: Client never sees PHP code, only output

Full Transcript

When a client requests a PHP file, the web server receives the HTTP request and checks the file extension. If it is a PHP file, the server passes the request to the PHP engine. The PHP engine reads and executes the PHP code inside the file. The code runs on the server and generates output, such as HTML text. This output is sent back to the web server, which then sends it as the HTTP response to the client. The client only sees the output, not the PHP code itself. Execution ends after the response is sent.