PHP验证信用卡卡号是否正确函数

可以使用以下PHP函数，验证一个卡号是否是信用卡：

function validateCard ( $cardnumber ) {    $cardnumber = preg_replace ( " //D|/s/ " , "" , $cardnumber ) ; # strip any non-digits    $cardlength = strlen ( $cardnumber ) ;   if ( $cardlength != 0 )    {      $parity = $cardlength % 2 ;     $sum = 0 ;     for ( $i = 0 ; $i < $cardlength ; $i ++ )      {        $digit = $cardnumber [ $i ] ;       if ( $i % 2 == $parity ) $digit = $digit * 2 ;         if ( $digit > 9 ) $digit = $digit - 9 ;           $sum = $sum + $digit ;     }      $valid = ( $sum % 10 == 0 ) ;     return $valid ;   }    return false ;}