Warning: A non-numeric value encountered in … How to solve it?

Question

When I migrated my app on a new server with PHP 7.2, the following message is showing up "Warning: A non-numeric value encountered in…"

The problem comes from this line (in the middle of the following code) $out = $out + strpos($index, substr($in, $t, 1)) * $bcp;.

How to solve it ? Thanks !

public function alphaID($in, $to_num = false, $pad_up = false, $pass_key = null)
{
    $out = '';
    $index = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $base = strlen($index);

    if ($pass_key !== null) {
        // Although this function's purpose is to just make the ID short

        for ($n = 0; $n < strlen($index); $n++) {
            $i[] = substr($index, $n, 1);
        }

        $pass_hash = hash('sha256', $pass_key);
        $pass_hash = (strlen($pass_hash) < strlen($index) ? hash('sha512', $pass_key) : $pass_hash);

        for ($n = 0; $n < strlen($index); $n++) {
            $p[] = substr($pass_hash, $n, 1);
        }

        array_multisort($p, SORT_DESC, $i);
        $index = implode($i);
    }

    if ($to_num) {
        // Digital number  <<--  alphabet letter code
        $len = strlen($in) - 1;

        for ($t = $len; $t >= 0; $t--) {
            $bcp = pow($base, $len - $t);
            $out = $out + strpos($index, substr($in, $t, 1)) * $bcp;
        }

        if (is_numeric($pad_up)) {
            $pad_up--;

            if ($pad_up > 0) {
                $out -= pow($base, $pad_up);
            }
        }
    } else {
        // Digital number  -->>  alphabet letter code
        if (is_numeric($pad_up)) {
            $pad_up--;

            if ($pad_up > 0) {
                $in += pow($base, $pad_up);
            }
        }

        for ($t = ($in != 0 ? floor(log($in, $base)) : 0); $t >= 0; $t--) {
            $bcp = pow($base, $t);
            $a = floor($in / $bcp) % $base;
            $out = $out . substr($index, $a, 1);
            $in = $in - ($a * $bcp);
        }
    }

    return $out;
}