Unique Learner Number (ULN) Validation in PHP

ulnValidation.php

<?php
/**
 * @method ulnValidation
 *
 * Checks if a ULN number is in the correct format, 10 digits including a 1 digit checksum
 *
 * @link  https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/795483/WSLP02_ULN_Validation_v3.pdf
 *
 * 1. Take the first 9 digits of the entered ULN.
 * 2. Sum 10 × first digit + 9 x second digit + 8 x third digit + 7 x fourth digit + 6 x fifth digit
 * + 5 x sixth digit + 4 x seventh digit + 3 x eighth digit + 2 × ninth digit
 * 3. Divide this number by 11 and find the remainder (modulo function).
 * 4. Subtract the remainder from 10. If this number matches the
 * tenth digit from the entered ULN, the ULN format is valid.
 * NOTE: the earlier versions of the gov document are wrong and it can be 0
 *
 * @param int $uln
 *
 * @return bool
 */
function ulnValidation($uln)
{

    // uln must only contain a number
    if (!is_numeric($uln)){
        echo "not numeric $uln";
        return false;
    }

    // uln must only be 10 digits
    if (10 != strlen(trim($uln))){
        echo "not 10 digits " . strlen(trim($uln));
        return false;
    }

    // uln must be positive whole number
    if (intval($uln) < 1) {
        echo "not positive number: " . intval($uln);
        return false;
    }

    // uln must only be a whole number
    if ($uln != round($uln)) {
        echo "not whole number: " . round($uln);
        return false;
    }

    // calculate the checksum
    $firstNine =  str_split(substr(trim($uln), 0, 9));
    $givenChecksum  = substr(trim($uln), -1);
    $total = 0;

    for ($i = 0; $i<9; $i++){
        $multiplier = 10-$i;
        $digit = intval($firstNine[$i]);
        $total += $multiplier * $digit;
    }

    $mod =  $total % 11;
    $calcChecksum = 10 - $mod;

    if ($givenChecksum != $calcChecksum){
        echo "checksum not valid $givenChecksum != $calcChecksum";
        return false;
    }

    return true;
}

?>