mod11.php

<?php

/**
 * Calculate the mod11
 * @param  string $baseVal
 * @param  string $separator
 * @return string
 */
public function mod11($baseVal = "", $separator = '-')
{
    $result = "";
    $weight = [ 2, 3, 4, 5, 6, 7,
                2, 3, 4, 5, 6, 7,
                2, 3, 4, 5, 6, 7,
                2, 3, 4, 5, 6, 7 ];

    /* For convenience, reverse the string and work left to right. */
    $reversedBseVal = strrev($baseVal);
    for ($i=0, $sum=0; $i < strlen($reversedBseVal); $i++) {
        /* Calculate product and accumulate. */
        $sum += substr($reversedBseVal, $i, 1) * $weight[$i];
    }

    /* Determine check digit, and concatenate to base value. */
    $remainder = $sum % 11;
    switch ($remainder) {
        case 0:
        case 1:
            $result = "{$baseVal}{$separator}0";
            break;

        default:
            $checkDigit = 11 - $remainder;
            $result = "{$baseVal}{$separator}{$checkDigit}";
            break;
    }

    return $result;
}