Why doesn't the following code work?

Code:
<?php
$levels_down = 1;
$read_length = 8192;
$inc_file = 'classes.txt';
$prefix = '';
for($i = 0; $i < $levels_down;$i++) {
    $prefix .= '../';
}

$filename = $prefix.$inc_file;

$lines = file($filename);
$search = 'classes';
$replace = $prefix . $search;
foreach($lines as $line) {
...