So, What is PHP then? 
PHP (Hypertext Preprocessor) is a widely-used scripting language which is especially 
  suited to Web Development. PHP can be embedded into HTML (Hypertext Markup language) 
  and used to generate dynamic content to web pages. PHP is similar to other Server-Side 
  scripting languages like ASP, Coldfusion, Perl, ASP, JSP etc.. but we think 
  it is superior to any of the others. PHP is open source which means it's free 
  to use and the source code behind it is also available. For more information 
  on PHP we recommend looking at the official PHP site at www.php.net.
If you are new to PHP why not check out some of the PHP 
  Tutorials on PHP-Editors. If you have any questions, feel free to drop into 
  our PHP forums and ask our 
  experts for help. 
Now, what is a PHP Editor or PHP IDE? 
Well, simply put, a PHP Editor is a computer program which is used to write  PHP Scripts. Some people use widely available tools like notepad on windows 
  or VI on unix. But if you want some extra features like code highlighting, code 
  insight, project management, code browser, code debugging etc, then a PHP IDE 
  (Integrated Development Environment) is more suited. There are hundreds available, some can be pretty expensive while some totally free. 
OK, what is this site all about then?
We brought together this site to try and help people find an Editor/IDE that 
  is suited to their needs and budget. Please browse around the PHP 
  Editor list we have put together and read what we, and the users, have to say.