Still about Collatz conjecture

<?php
for ($i = 1; $i > 0; $i++) {
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    $s .= $a[0] . " ";
    for ($j = 1; $j > 0; $j++) {
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        if ($a[$j-3] . $a[$j-2] . $a[$j-1] == "421") { $j = -1; }
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}
?>