PHP ID number verification function_PHP tutorial

function validation_filter_id_card($id_card)
{
if(strlen($ id_card) == 18)
{
return idcard_checksum18($id_card);
}
elseif((strlen($id_card) == 15))
{
$id_card = idcard_15to18($id_card);
return idcard_checksum18($id_card);
}
else
{
return false;
}
}
// Calculate ID card Verification code, according to the national standard GB 11643-1999
function idcard_verify_number($idcard_base)
{
if(strlen($idcard_base) != 17)
{
return false;
}
//Weighting factor
$factor = array(7, 9, 10, 5, 8, 4, 2, 1, 6, 3, 7, 9, 10, 5, 8, 4, 2 );
//Verification code corresponding value
$verify_number_list = array('1', '0', 'X', '9', '8', '7', '6', '5 ', '4', '3', '2');
$checksum = 0;
for ($i = 0; $i < strlen($idcard_base); $i++)
{
$checksum += substr($idcard_base, $i, 1) * $factor[$i];
}
$mod = $checksum % 11;
$verify_number = $verify_number_list[$ mod];
return $verify_number;
}
// Upgrade the 15-digit ID card to 18-digits
function idcard_15to18($idcard){
if (strlen($idcard) != 15){
return false;
}else{
// If the ID card sequence code is 996 997 998 999, these are special codes for people over 100 years old
if (array_search(substr( $idcard, 12, 3), array('996', '997', '998', '999')) !== false){
$idcard = substr($idcard, 0, 6) . ' 18'. substr($idcard, 6, 9);
}else{
$idcard = substr($idcard, 0, 6) . '19'. substr($idcard, 6, 9);
}
}
$idcard = $idcard . idcard_verify_number($idcard);
return $idcard;
}
// 18-digit ID verification code validity check
function idcard_checksum18($idcard){
if (strlen($idcard) != 18){ return false; }
$idcard_base = substr($idcard, 0, 17);
if (idcard_verify_number( $idcard_base) != strtoupper(substr($idcard, 17, 1))){
return false;
}else{
return true;
}
}