php script help!

[nekng]

ok, so i want to make a php secipt that will find some code on another page and edit it, how ever i have been unable to find a totural that explanes what to do, so could some on help me?(give me a code, like:
open="file to cahnge"
find="code to cahnge"
replace="the new code" , you could also link me to a totural but the first method would be preferd)