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.

Practice

1. What happens when a PHP file is requested by a browser?

easy

A. The PHP code is downloaded as a file to the browser

B. The browser runs the PHP code directly

C. The server runs the PHP code and sends the output to the browser

D. The server sends the PHP code as plain text to the browser

Solution

1. Step 1: Understand PHP execution location

   PHP code runs on the server, not in the browser.

2. Step 2: Identify what the browser receives

   The server sends only the output (usually HTML) to the browser, not the PHP code itself.

3. Final Answer:

   The server runs the PHP code and sends the output to the browser -> Option C

4. Quick Check:

   PHP runs on server = A [OK]

Hint: Remember: PHP runs on server, browser gets output only [OK]

Common Mistakes:

• Thinking browser executes PHP code
• Expecting PHP code to be visible in browser source
• Confusing PHP with client-side languages like JavaScript

2. Which of the following is the correct way to start a PHP block in a file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall PHP opening tags

   The standard and recommended way to start PHP code is with

2. Step 2: Check each option

   uses which is correct. Others are invalid or incomplete tags.

3. Final Answer:

   -> Option A

4. Quick Check:

   Standard PHP tag = B [OK]

Hint: PHP code always starts with php [OK]

Common Mistakes:

• Using short tags like which may be disabled
• Confusing PHP tags with HTML or script tags
• Missing the 'php' keyword after

3. What will be the output of this PHP code when accessed via a browser?

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

medium

A. Syntax Error

B. echo "Hello, " . "world!";

C. Hello, . world!

D. Hello, world!

Solution

1. Step 1: Understand echo statement

   The echo command outputs the combined string "Hello, world!" to the browser.

2. Step 2: Confirm string concatenation

   The dot operator joins "Hello, " and "world!" correctly without errors.

3. Final Answer:

   Hello, world! -> Option D

4. Quick Check:

   echo concatenates strings = D [OK]

Hint: echo prints combined strings directly [OK]

Common Mistakes:

• Expecting PHP code to show in browser
• Confusing concatenation operator
• Thinking dot prints as text

4. Identify the error in this PHP code snippet:

<?php
echo "Welcome to PHP!"
?>

medium

A. Missing quotes around string

B. Missing semicolon after echo statement

C. Incorrect PHP opening tag

D. Using echo instead of print

Solution

1. Step 1: Check syntax of echo statement

   PHP statements must end with a semicolon ; to separate commands.

2. Step 2: Locate missing semicolon

   The echo line lacks a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after echo statement -> Option B

4. Quick Check:

   Statements need ; = C [OK]

Hint: Always end PHP statements with a semicolon ; [OK]

Common Mistakes:

• Forgetting semicolon at line end
• Confusing PHP tags with HTML tags
• Assuming echo doesn't need semicolon

5. You want to create a PHP page that shows the current year dynamically. Which code snippet correctly does this?

<?php
// Options:
// A: echo date("Y");
// B: echo "date(Y)";
// C: echo date(Y, Y);
// D: echo "Y";
?>

hard

A. echo date("Y");

B. echo "date(Y)";

C. echo date(Y, Y);

D. echo "Y";

Solution

1. Step 1: Understand PHP date function usage

   The date function requires the format string in quotes to return the current year.

2. Step 2: Evaluate each option

   echo date("Y"); correctly calls date("Y") to output the year. echo "date(Y)"; prints the string literally. echo date(Y, Y); misses quotes causing undefined constants and wrong output (1970). echo "Y"; prints letter Y only.

3. Final Answer:

   echo date("Y"); -> Option A

4. Quick Check:

   Correct date format string = A [OK]

Hint: Use quotes around format in date() function [OK]

Common Mistakes:

• Omitting quotes around format string
• Printing function call as string
• Confusing output with literal text