PHP stands for "PHP: Hypertext Preprocessor" and is a Server Side Language which means is runs on the Server and doesn't run on the users Browser. PHP is an extremely power language and is used to manipulate all sorts of information from database, other websites or whatever it's supplied and convert this back into HTML based code that is sent to the users browser for interpretation and viewing.
Web Hosting PHP is offered with most Website Hosting Packages and PHP is a very common Server Side language meaning the level of support on the Internet is very strong - eg: if you have a problem you can almost always find a solution on Google. Apache PHP are like Brother and Sister and run perfectly in tandem. Apache offers a great web server as a PHP Development Environment.
If you need PHP work done there are more than enough PHP programmers online and you can always hire a PHP Programmer to develop or extend any solution. PHP takes a little time to learn but it's well worth the time. Once you have the skills you will able to enhance PHP Web Developments in countless ways.
Why would you choose PHP over other Server Side Languages like ASP, ASP.NET, Ruby on Rails etc. PHP runs on Linux which is much more stable and not to mention free then ASP Windows Servers and the level of support for PHP from other PHP Programmers provides an incredibly stable environment for troubleshooting.
PHP also comes with many different frameworks from Cake PHP, to Codeigniter, PHP Nuke, Moo Tools and many more. The advantage of frameworks is they allow faster code development because they have functions in place to get well used functions done quickly like a join form. The disadvantage is you learn how to code in the framework rather than the underlying language and therefore if the framework was to ever be discontinued or you ended up in a situation where the framework wasn't available you would need to learn to code in the raw language yourself. I would advise coding in raw PHP yourself rather than hire PHP programmer or use a PHP framework like Cake PHP.
One of the best resources on PHP is www.php.net. Here you will find every PHP function and examples of how to use them. You can also check for deprecated functions that should be avoided and look for the best ways to get things done. Finally remember to check your PHP Version on your local system and PHP Web Hosting Solution so you know what functions are available. For a local PHP Development Environment you best to go with WAMP or LAMP and check your php.ini to see what modules you have configured by default.