<?php

$f1 = file('file1.txt');
$f2 = file('file2.txt');
$new_data = array();

foreach ($f2 as $entry) {
    $data = explode(':', $entry);
    if (!in_array($data[0], $f1) {
        $new_data[] = $entry;
    }
}

$f3 = fopen('file2.txt', 'w+');
fwrite($f3, implode(PHP_EOL, $new_data);
fclose($f3);