PHP is a general purpose server-side scripting language that was written specifically for web developers. Many other languages used for web development existed before the advent of the internet and are therefore not necessarily so well suited for the purpose.
It is much easier to use than many other server-side languages due to it's embedded nature. Snippets of PHP code can be placed within your HTML page and the server processes these before outputting a plain HTML page to the browser. This means it is extremely easy for beginners to pick up and master the basics.
PHP is supported by dedicated developers, a massive online community and thousands of ready-made scripts. Current statistics estimate that over 13 million domains worldwide use PHP (source php.net).