witing to a file

[iotas]

i have a file i wish to write to and i need to replace 1 line.
could some one give me some code to do this.
here is the file

<?php
$passcode = '7bef9410735ba070fb055f8a35ede02d';
?>

i need some code that will replace the 7bef9410735ba070fb055f8a35ede02d with another code when i change the passcade.
any help would be apreciated.

regards onic_iota

Thanks in advanced

[weekender]

it would be easier to overwrite the file and write all three lines to it, rather than just edit one line.