Language filter

[evank3]

First off I need to know how to write to a certain line of a file from a form... I have a code here, but do not know how to add it to a certain line:
[code]<?php
$word = $_REQUEST["word"] ;
$writedata = $word . "\n";
$file = fopen("filter.txt", "a");
fwrite($file, $word);
fclose($file);
?>[/code]
I need "fwrite($file, $word);" to write the word to line 4 of  my other file filter.php

Next I need to be able to delete certain words from line 4 of filter.php from a form.

Thanks in advance,
Evan