reading/editing php file with php script

Hello; I have been writing a php script which is supposed to open and edit a php file on the same server, in the same directory. But I have not been able to get regex as in function ereg() to work to match a line of code to alter. I also have not been able to get strpos() to work in this code. I am not complaining, i am just curious if this is a security feature of php, that it can't be used to open and edit existing script files on a web server. Or am I doing something wrong? I have successfully used php to write a php script file from scratch and then include it. It works good for keeping track of things on the server. Thanks JK