Php id card number verification function

The function for verifying the ID card number in php is quite complete. all old people are included. The code is as follows:

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 the ID card verification code based on 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 );
// Check code 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-digit
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 elderly people over years old
If (array_search (substr ($ idcard, 12, 3), array ('20160301', '20160301', '20160301 '))! = 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;
}
// Check the validity of the 18-digit ID card verification code
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;
}
}