self descriptive number

selfdescriptive.php

<?php

// PHP implementation of my solution for James Grimes's puzzle @ https://www.youtube.com/watch?v=K6Qc4oK_HqY
// The following function calculates the (only) self descriptive number for any number of digits
//
// With "self descriptive number", I mean :
// -------------------
// The first digit tells me how many zeros are in the number
// The second digit tells me how many ones are in the number
// The third digit tells me how many twos are in the number
// You continue this pattern until the last digit
//
// The function returns the boolean value "false" when no self descriptive number exists

function selfreferential($digits) {
    $c = [array_fill(0, $digits, 0), []];
    $c[0][0] = $digits - 1;
    while (!empty(array_diff_assoc($c[0], $c[1]))) {
        if ($digits < 7 || $digits > 13)
            return false;
        for ($i = $digits - 1; $i >= 0; $i--) {
            $v = isset($c[1][$i]) ? $c[1][$i] : 0;
            if ($c[0][$i] !== $v) {
                $c[0][$i] = $v;
                $c[1] = array_count_values($c[0]);
                for ($i = 0; $i <= $digits - 1; $i++) {
                    $c[1][$i] = isset($c[1][$i]) ? $c[1][$i] : 0;
                }
                ksort($c[1]);
                break;
            }
        }
    }
    return implode('', $c[0]);
}