XY123456(Z)

var $validate = array(
     'hkid' => array(
        'checkDigit' => array(
            'rule' => 'checkDigit',
            'message' => '請填寫正確的香港身份證號碼。<br />Please input a valid HKID.'
        )
    )
);
 
function checkDigit($hkid){
    // the value is stored in $hkid['hkid'], store it back to $hkid for ease of use
    $hkid = $hkid['hkid'];
 
    // create an array for the weight of the letters
    $weight = array();
    for ($i=65; $i&lt;91; $i++){
        $weight[chr($i)] = ($i-65)%11+1;
    }
 
    // add a space with weight 0 for HKID without the first letter
    $weight[' '] = 0;
 
    if ((strlen($hkid) == 8) || (strlen($hkid) == 9)){
        // convert all letters to uppercase
        $hkid = strtoupper($hkid);
        // make all HKID become 9-digit long
        if (strlen($hkid) == 8){
            $hkid = ' '.$hkid;
        }
        // split the HKID string into a 9-element long array
        $chars = preg_split('//', $hkid, -1, PREG_SPLIT_NO_EMPTY);
 
        $value = 0;
        // calculate the sum with the 9-position value
        $value += $weight[$chars[0]]*9;
        $value += $weight[$chars[1]]*8;
 
        for ($i=2; $i&lt;8; $i++){
            $value += $chars[$i]*(9-$i);
        }
 
        // compare the check digit value and the calculated 11 - 11-modulus 
        if (is_numeric($chars[8])){
            $checkValue = $chars[8];
        }
        elseif ($chars[8] == 'A'){
            $checkValue = 10;
        }
        return ($checkValue == (11 - $value%11));
    }
    return false;
}