The reason? Someone left out the exit; after the Header Location call, which allows the execution to continue down the page. Moral: Make sure you call exit after a PHP Header call to Location if you want execution to branch immediately!

if (InvalidInput($Stuff))
{
header("Location: http://www.example.com/"); /* Redirect browser */
exit; /* Make sure that code below does not get executed when we redirect. */
}
/* Execute on valid input */
?>

PHP is a powerful server-side scripting language for creating dynamic and interactive websites. Server side means that it runs on the server hosting the page, rather than the client or browser. PHP is widely used, partly because it is free, but mainly because it provides a fast and efficient alternative to competitors such as Microsoft’s ASP. It also supports database access easily and with much fewer lines of code than ASP. PHP is perfectly suited for Web development and can be embedded directly into the HTML code.

One place to start is the PHP manual, which unsurprisingly has a Getting Started section. My advice is to scan read as much of this as you can before attempting to start programming in PHP. It will explain what PHP is and can do, and then gives you a simple tutorial.

Click here to visit PHP manual

Another good place to start learning PHP is the PHP Tutorial on the w3schools website. This provides a complete references of all PHP functions and some user friendly cross links to functions that you may not even realise that you will need.

Click here to visit W3Schools PHP section

If you are new to PHP programming check out Getting started with PHP