Is there a special directory in which php scripts are to be placed

Hi, I am asking this question because when I give a "script alias" for the php script the server gives 500 Interval Server Error while trying to execute the script.But when i don't give script alias and try to execute it in htdocs directory it executes perfectly. When I asked my colleague, he told me that php scripts are basically html files. Therefore, the script alias won't work for php scripts. I want to know if this statement is correct. If not, how do I initialize a script alias for the php scripts. Thanks Manas