if (!file_exists('3.mp3'))
{
$first_fp = fopen('1.mp3', 'rb+');
$c = fread($first_fp, filesize('1.mp3'));
fclose($first_fp);

$scnd_fp = fopen('2.mp3', 'rb+');
$c .= fread($scnd_fp, filesize('2.mp3'));
fclose($scnd_fp);

fclose(fopen('3.mp3', 'a+'));

$thrd_fp = fopen('3.mp3', 'r+');
fseek($thrd_fp, 0);
ftruncate($thrd_fp, 0);
fwrite($thrd_fp, $c);
fclose($thrd_fp);
echo 'ok';
}
else
{
echo 'file_exists';
}