PHP

What is PHP?

PHP is a recursive acronym for PHP: Hypertext Preprocessor. It is a server-side scripting language whose main purpose is to generate the HTML that is viewed by the client. The PHP code itself is never visible to the client: if you choose View Source in your browser you see only the resulting HTML, not the PHP code. This is different from client-side languages such as JavaScript, whose code is downloaded to and visible in the browser.

PHP was created by Rasmus Lerdorf in 1995 and saw its first official releases soon after. Today a very large share of the world's web sites – including WordPress, Wikipedia (MediaWiki) and many large-scale applications – run on PHP.

PHP in short

• PHP stands for PHP: Hypertext Preprocessor.
• PHP is a server-side scripting language, like ASP.NET or JSP.
• PHP scripts are executed on the server; the browser receives plain HTML.
• PHP supports many databases – MySQL/MariaDB, PostgreSQL, Oracle, SQLite, Microsoft SQL Server, and any database through ODBC.
• PHP is open source software and is free to download and use.

What is a PHP file?

• A PHP file can contain text, HTML tags and PHP scripts.
• PHP files are returned to the browser as plain HTML.
• PHP files have the extension .php. (Very old servers also accepted .php3 and .phtml.)

Why PHP?

• PHP runs on many platforms – Windows, Linux, Unix, macOS.
• PHP is compatible with almost all web servers in use today – Apache, Nginx, IIS.
• PHP is free to download from the official resource, www.php.net.
• PHP is easy to learn and runs efficiently on the server side.

Basic PHP Syntax

A PHP scripting block always starts with <?php and ends with ?>. A scripting block can be placed anywhere in the document.

<?php
    statement;
    statement;
?>

A PHP file normally contains HTML tags, just like an HTML file, plus some PHP scripting code. Below is a simple PHP script that sends the text "Hello World" to the browser:

<html>
<body>
<?php
echo "Hello World";
?>
</body>
</html>

Hello World

• Each code line (statement) in PHP must end with a semicolon. The semicolon is a separator that distinguishes one instruction from another. The semicolon before ?> may be omitted, but it is good practice to write it.
• There are two basic statements to output text with PHP: echo and print.
• The file must have a .php extension. If the file has a .html extension, the PHP code is not executed and the browser simply shows the text.

Output with echo and print

Both echo and print are language constructs (not real functions), so parentheses are optional.

<?php
echo "<h3>Using echo and print</h3>";
echo "Hello ", "World", "!<br>";          // echo accepts many arguments
echo("PHP is fun.<br>");                  // parentheses are optional
print "print outputs one string.<br>";
$r = print "print also returns a value: ";
echo $r . "<br>";
$x = 10;
$y = 20;
echo "Sum of $x and $y is " . ($x + $y);
?>

Using echo and print
Hello World!
PHP is fun.
print outputs one string.
print also returns a value: 1
Sum of 10 and 20 is 30

Mixing PHP and HTML

PHP can switch in and out of HTML as often as needed. Everything outside <?php ... ?> is sent to the browser unchanged.

<?php $title = "Courses"; $fee = 4500; ?>
<!DOCTYPE html>
<html>
<head><title><?= $title ?></title></head>
<body>
  <h2><?= $title ?> offered this month</h2>
  <ul>
    <li>Core PHP – fee Rs. <?= $fee ?></li>
    <li>Advance PHP – fee Rs. <?php echo $fee + 1500; ?></li>
  </ul>
  <p>Page generated on <?= date("d-m-Y") ?>.</p>
</body>
</html>

Courses offered this month
Core PHP – fee Rs. 4500
Advance PHP – fee Rs. 6000

Page generated on 22-10-2026.

The browser receives only HTML. If you choose View Source you will see, for example, <li>Core PHP – fee Rs. 4500</li> – the PHP code has already been replaced by its result.

Comments in PHP

Comments are ignored by PHP. Use them to explain your code to other programmers – and to yourself months later.

• // – single-line comment (C++ style).
• # – single-line comment (shell style).
• /* ... */ – multi-line comment block (C style).

<html>
<body>
<?php
// This is a single-line comment
# This is also a single-line comment
/*
   This is
   a comment
   block
*/
echo "Only this line is displayed."; // comment after code
?>
</body>
</html>

Only this line is displayed.

Case Sensitivity and White Space

• Keywords, function names and class names are not case-sensitive: ECHO, echo and Echo are the same; STRLEN() calls strlen().
• Variable names are case-sensitive: $name, $Name and $NAME are three different variables.
• Constants defined with define() are case-sensitive.
• Extra spaces, tabs and new lines between tokens are ignored, so you can format code for readability.

<?php
$color = "red";
ECHO "My car is " . $color . "<br>";
echo "My house is " . $COLOR . "<br>";
Echo "Length of 'PHP' = " . STRLEN("PHP");
?>

My car is red

Warning: Undefined variable $COLOR in /opt/lampp/htdocs/phpbook/ch03/case.php on line 4
My house is
Length of 'PHP' = 3

The first and last lines work, because ECHO, Echo and STRLEN are case-insensitive. The second line shows a Warning because $COLOR is a different – undefined – variable from $color.

Omitting the Closing Tag

If a file contains only PHP code, the closing ?> at the end of the file may be omitted. This is recommended for pure-PHP files (classes, configuration files) because it prevents accidental white space after ?> from being sent to the browser – which can break header(), setcookie() and session_start() (see Chapter 31).