PHP (recursive acronym for “PHP: Hypertext Preprocessor”) is a widely-used Open Source general-purpose scripting language that is especially suited for Web development and can be embedded into HTML.
Simple answer, but what does that mean? An example:
An introductory example
<html>
<head>
<title>Example</title>
</head>
<body>
<?php
echo "Hi, I'm a PHP script!";
?>
</body>
</html>