How would I alter a file?

[BeatificFaith]

Hi, I am making my first plugin and here is my question... How would I go about replacing/altering text in a file? For instance if I wanted to change the below in profile.php:

PHP Code:

'year' => INT,
[change to]-->
'year' => INT, 'position' => INT, 


?
Do I use the normal PHP functions for opening and writing to a file? I'm asking so I don't mess up any on my forum (albeit a test forum).
Thank you

Nevermind, I figured it out that I need to use the substring functions.