PHP is a server side open source scripting language. Its stand for Personal Home Page, it is also known as Hypertext Preprocessor. It is used for web development , approximately 240 million websites has been developed using PHP, Hundred s of (content Management System) CMS has been made using PHP. some well-known CMS listed below.
1.  Drupal
2. Magento
3. Joomla
4. WordPress
5. concrete5
6. PHP-Fusion
The list are very large but due to shortage of time I only mentioned the popular CMS only.

Why PHP is so Popular?

Most of the people ask me that why PHP is so popular than other language, my answer is that,
1.    PHP is simple and open source and therefore lot of helps are already exist on internet.
2.    It is easy to use.
3.    PHP code can easily embed into html. For example if you want to make html page dynamic then you can easily convert it by changing the extension from .html to .PHP and then embed your PHP code.

Example code
<html >
<head>
<title>PHP embed</title>
</head>
 If you put a dynamic heading using PHP . just do in such a way. For example purpose I store a title into a PHP variable and print that title within h2 tag.

 <?php

$title    =    “embedding php into html”;
?>
<h2><?php echo $title;?></h2>

 <body>
</body>
</html>

 now just save it into local server using .php extension an run.