PHP introduction: what PHP is, how server-side code works, who uses it and setting up your computer

PHP is a programming language that runs on the web server and builds the pages your browser receives. When you log in to a school portal, submit a contact form, check out on a small online shop or read a WordPress blog, there's a good chance PHP did the work: it checked your password, saved the form to a database, calculated the total, or put the blog post into a page.

PHP powers WordPress (which runs a very large share of all websites), Laravel apps, Moodle (used by many universities for e-learning), and countless Kenyan school, SACCO, hospital and shop systems. It runs on almost every cheap shared hosting plan (cPanel), which makes it one of the most practical languages for freelancers building websites for local clients.

Client-side vs server-side

You need both: HTML/CSS/JavaScript for what users see and interact with, and a server-side language for anything that must be stored, secured or trusted.

What happens when you visit a PHP page

1. The browser asks the server for https://shop.example.co.ke/products.php?category=phones.
2. The web server (Apache or Nginx) sees .php and hands the file to PHP.
3. PHP runs the code: reads the category value, queries the MySQL database for phones, builds an HTML list.
4. The server sends back plain HTML. The visitor never sees your PHP code, database password or business logic.
5. The browser displays the page.

Each request starts fresh: PHP runs, produces output, and finishes. Data that must survive between pages is kept in a database, sessions or cookies (later lessons).

Who uses PHP

Jobs: PHP developer, WordPress developer, Laravel developer, full-stack developer, back-end developer. Many freelance clients in Kenya want "a website with a system": PHP + MySQL is a direct path to that work.

Your first PHP code

Press Run to execute PHP right here:

<?php
echo "Habari, Kenya!\n";
echo "Today is " . date("l") . ".\n";
echo "2 + 3 = " . (2 + 3) . "\n";

Syntax rules:

• PHP code starts with <?php. In a file that's only PHP, leave out the closing ?> (it avoids accidental whitespace being sent).
• Every statement ends with a semicolon ;.
• echo outputs text; . joins strings (concatenation).
• "\n" is a new line in text output (in a web page you'd use <br> or HTML elements).
• Variable names start with $: $name.
• PHP keywords and function names aren't case-sensitive, but variable names are: $Name and $name are different.

Mixing PHP with HTML

A .php file can contain HTML with PHP blocks inside:

<!DOCTYPE html>
<html>
<body>
  <h1><?php echo "Habari, Kenya!"; ?></h1>
  <p>Today is <?= date("l, j F Y") ?></p>
  <?php $items = ["Laptop bag", "Wireless mouse", "Phone charger"]; ?>
  <ul>
    <?php foreach ($items as $item): ?>
      <li><?= htmlspecialchars($item) ?></li>
    <?php endforeach; ?>
  </ul>
</body>
</html>

• <?= ... ?> is a shortcut for <?php echo ... ?>.
• The foreach (...): ... endforeach; style is easier to read inside HTML.
• htmlspecialchars() makes text safe to put into HTML (the security lesson explains why this matters).

Comments

<?php
// A single-line comment
# Also a single-line comment
/* A multi-line
   comment */
echo "Comments are ignored by PHP\n";

echo vs print, and printf

<?php
$name = "Amina";
$balance = 15250.5;
echo "Hello, $name\n";                       // variables inside double quotes are replaced
print "print works too\n";
printf("Balance: KSh %s\n", number_format($balance, 2));
printf("%-10s|%5d|\n", "Chapati", 30);       // formatted columns

Errors

PHP tells you what went wrong and on which line. Learn to read the messages:

<?php
// fails on purpose: a missing semicolon on the next line
echo "Hello"
echo "World";

During development, show errors; on a live site, log them instead of showing them to visitors (they can reveal file paths and other details attackers use).

Setting up PHP on your computer

Editor: VS Code with a PHP extension (e.g. PHP Intelephense) gives colour-coding, auto-complete and error hints.

How this subject is organised

1. Language basics: variables, strings, numbers, arrays, conditions, loops, functions, OOP.
2. Web basics: forms, validation, security.
3. Databases: MySQL with PDO, a full CRUD app.
4. Real features: sessions and logins, file uploads, JSON APIs, M-Pesa payments.
5. A complete project: a booking system.

Summary

• PHP is a server-side language: it runs on the server and sends HTML to the browser; visitors never see the code.
• It powers WordPress, Laravel apps, Moodle and many local business systems, and runs on almost all shared hosting.
• Code starts with <?php, statements end with ;, variables start with $, echo outputs, . joins strings.
• PHP can be mixed with HTML using <?php ?> and <?= ?>; use htmlspecialchars() when printing text.
• Set up with XAMPP, Laragon or php -S; read error messages carefully and hide them on live sites.

Learn more: PHP: The Right Way · Official PHP manual