Collatz conjecture (command line PHP windows)

<?php
$input = readline("Up to... ? ");
$s = "";
for ($i = 1; $i <= $input; $i++) {
    $a[0] = $i;
    $s .= $a[0] . " ";
    for ($j = 1; $j > 0; $j++) {
        $a[$j] = ($a[$j-1] % 2)*(3*$a[$j-1] + 1) + (1 - ($a[$j-1] % 2))*($a[$j-1] / 2);
        $s .= $a[$j] . " ";
        if ($a[$j-3] . $a[$j-2] . $a[$j-1] == "421") { $j = -1; }
    }
    $s .= "\n";
    print $i . " done\n";
}
file_put_contents("collatz.log", $s);
?>