PHP验证信用卡卡号是否正确函数

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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 ;

}