kaprekar_constant.php

<?php

function kaprekar_constant($number)
{
    $constant = 6174;
    if (strlen((string)$number) == 3) {
        $constant = 495;
    }

    do {
        $arrays = array_map('intval', str_split($number));

        rsort($arrays);
        $reversed = implode('', $arrays);

        sort($arrays);
        $sorted = implode('', $arrays);

        $number =  $reversed - $sorted;
    } while ($number != $constant);
}