The PHP Include Path is a set of locations that is used for finding resources referenced by include/require statements.
Elements added to a project's Include Path affect the following:
Running/Debugging
Go to source
                    
Content Assist - Adding projects/libraries to a project's Include Path will make elements defined within the included projects/libraries available as Content Assist options to the project.
In 'include'/'require' calls, file locations can be defined in three ways:
Absolute Path- The exact file location 
	 is specified (e.g. C:\Documents and Settings\MyProject\myfolder\a.php). 
	 During 
Relative to the Current Working Directory - File names preceded with a "./" or a "../" These will only be searched for relative to the PHP 'Current Working Directory'. You can find out the location of your Current Working Directory by running the command "echo getcwd()".
Relative Path - Only the file name or partial 
	 path is specified (e.g. /myfolder/a.php). In this case, Zend Studio will search for 
	 the file's path according to the resources and order configured in 
	 the project's Include Path.
	If the path of the file being searched for exists in more than one 
	 location, the file that is called will be the first one Zend Studio encounters during 
	 the search process.
Projects in the "debug target" (the first file to be debugged) project's Include Path, according to the order in which they are listed. If a project specified in the Include Path refers to other projects/libraries in its own Include Path, the file path will be searched for there before the search process continues.
The "debug target" file's project.
See http://il2.php.net/manual/en/function.include.php for more on PHP's search mechanism.
