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Module 01: Hello World

Learning Objectives

Understand that PHP executes on the server, not in the browser
See how PHP output becomes the HTTP response body
Learn to mix PHP code with HTML (templating)
Understand that PHP runs fresh for each request

The Request-Response Cycle

When you request a .php file, here's what happens:

1. Browser sends HTTP request:  GET /hello.php HTTP/1.1

2. Web server receives request, sees .php extension

3. Server invokes PHP interpreter to execute the script

4. PHP runs the code, collecting all output (echo, print, HTML outside tags)

5. Server wraps output in HTTP response:
   HTTP/1.1 200 OK
   Content-Type: text/html

   Hello, World!

6. Browser displays the response

Demo Files

hello.php The simplest possible PHP script - just echoes text.

templating.php Mix PHP with HTML - dynamic content in static pages.

The Simplest PHP Script

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

That's it! The <?php tag starts PHP mode, echo outputs text, and ?> ends PHP mode.

Tip: For files that are pure PHP (no HTML), you can omit the closing ?> tag. This prevents accidental whitespace issues.

Mixing PHP and HTML

<!DOCTYPE html>
<html>
<body>
    <h1><?php echo "Hello, World!"; ?></h1>
    <p>Current time: <?= date('H:i:s') ?></p>
</body>
</html>

HTML outside <?php ?> passes through unchanged
<?= ... ?> is shorthand for <?php echo ... ?>
PHP is essentially a templating language at heart

Key Concept: Stateless Execution

PHP runs per-request. Each request starts fresh:

New PHP process (or thread) is created
Script executes from the beginning
All variables are created anew
Response is sent
Everything is discarded

This is why we need sessions to maintain state between requests.

Try This

View hello.php and note what appears in the browser
View templating.php and refresh several times - watch the time change
Right-click → "View Page Source" to see that no PHP code appears in the output
Try modifying the files and refreshing to see changes immediately

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