class.credit.php
<code lang="PHP">
<?php 

class credit{

	function credit(){
	
	}
	
	function validate($cc_number, $cvv2, $date){
		$this->validateCard($cc_number);
		$this->validateExDate($date);
		$this->validateCVV($cc_number, $cvv2);
	}

	function validateCard($cc_number){
		global $e;
		
		$first_number = substr($cc_number, 0, 1);
		switch ($first_number){
			case 3:
				if (!preg_match('/^3\d{3}[ \-]?\d{6}[ \-]?\d{5}$/', $cc_number)) $e->setError("American Express number is not valid");
				break;
			case 4:
				if (!preg_match('/^4\d{3}[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cc_number)) $e->setError("Visa number is not valid");
				break;
			case 5:
				if (!preg_match('/^5\d{3}[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cc_number)) $e->setError("MasterCard number is not valid");
				break;
			case 6:
				if (!preg_match('/^6011[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cc_number)) $e->setError("Discover Card number is not valid");
				break;
			default:
				 $e->setError("Credit Card number is not valid: Error 1");
				 break;
		}
		
		if($e->countErrors() == 0) $this->validateCard2($cc_number);

	}
	
	function validateCard2($cc_number){
		global $e;
		
		$checksum = 0;
		
		$j = 1;
		
		for ($i = strlen($cc_number) - 1; $i >= 0; $i--){
		
			$calc = substr($cc_number, $i, 1) * $j;
			
			if ($calc > 9){
				$checksum = $checksum + 1;
				$calc     = $calc - 10;
			}
			
			$checksum += $calc;
		
			if ($j == 1){
				$j = 2;
			}else{
				$j = 1;
			}
		}
		
		if ($checksum % 10 != 0) $e->setError("Credit Card number is not valid: Error 2");
	}
	
	function validateExDate($date){
		global $e;
		
		$month =  substr($date, 0, 2);
		$year =  substr($date, 2, 2);
		
		$current_month = date("m");
		$current_year  = date("y"); 
		if ($year < $current_year){ 
			$e->setError("Expiration Date is not valid : Error 3"); 
		}else{ 
			if ($year == $current_year)
			{
				if ($month < $current_month){
					$e->setError("Expiration Date is not valid : Error 4"); 
				}
			}
		}
	}
	
	function validateCVV($cc_number, $cvv2){
		global $e;
		
		$first_number = substr($cc_number, 0, 1);
		
		if ($first_number == 3){
			if (!preg_match("/^\d{4}$/", $cvv2)) $e->setError("CVV number is not valid : Error 5");
		}else{
			if (!preg_match("/^\d{3}$/", $cvv2)) $e->setError("CVV number is not valid : Error 6");
		}
	}
	
	function maskCard($cc_number){
		$count = strlen($cc_number);
		for($i = 1; $i <= ($count -4); $i++){
			echo "X";
		}
		echo substr($cc_number, -4);
	}

}

?>
</code>

test_credit.php
<code lang="PHP">
<?php 
	
	require_once "class.error.php";
    $e = new error;
	require_once "class.credit.php";
	$cc = new credit;	
	
	if($_POST){
		$cc_number = $_POST['cc_number'];
		$expire_month = $_POST['expire_month'];
		$expire_year = $_POST['expire_year'];
		$cc_cvv = $_POST['cc_cvv'];
		
		$date = $expire_month . $expire_year;
		$cc->validate($cc_number, $cc_cvv, $date);
		
		if($e->countErrors() == 0){
			//process form
			header("location: somewhere.php");
			die();
		}
	}
		
?>
    
    <?php $e->showErrors(); ?>
	<form method="post" action="<?php echo $_SERVER['PHP_SELF'] ?>">
    <table border="0" cellpadding="4" width="450">
      <tr>
        <td width="200" class="tbl_left">Payment Type: </td>
        <td width="250" class="tbl_right">Credit Card</td>
      </tr>
      <tr>
        <td class="tbl_left">Card Holder Name: </td>
        <td class="tbl_right"><input type="text" name="cc_name" /></td>
      </tr>
      <tr>
        <td class="tbl_left">Card Type: </td>
        <td class="tbl_right"><select name="cc_type">
        <option value="American Express">American Express</option>
        <option value="Discover">Discover</option>
        <option value="Master Card">Master Card</option>
        <option value="Visa">Visa</option>
    </select> 
    </td>
      </tr>
      <tr>
        <td class="tbl_left">Credit Card Number: </td>
        <td class="tbl_right"><input type="text" name="cc_number" /></td>
      </tr>
      <tr>
        <td class="tbl_left">Expiration Date: </td>
        <td class="tbl_right"><select name="expire_month">
    <option value="01">January</option>
    <option value="02">February</option>
    <option value="03">March</option>
    <option value="04">April</option>
    <option value="05">May</option>
    <option value="06">June</option>
    <option value="07">July</option>
    <option value="08">August</option>
    <option value="09">September</option>
    <option value="10">October</option>
    <option value="11">November</option>
    <option value="12">December</option>
    </select> 
    <select name="expire_year">
    <?php 
    for($x=date(Y); $x<=2050; $x++){
    echo '<option value="'.substr($x, -2).'">'.$x.'</option>'."\n";
    }
    ?>
    </select>
    </td>
      </tr>
      <tr>
        <td class="tbl_left">CVV Number:</td>
        <td class="tbl_right"><input type="text" name="cc_cvv" size="5" maxlength="4" /></td>
      </tr>  
      <tr>
        <td class="tbl_left">Enter a Promo Code: </td>
        <td class="tbl_right"><input type="text" name="promo_code" /></td>
      </tr>
    </table>
</form>
</code>

To mask a credit card number use this:
<code lang="PHP">
echo $cc->maskCard($cc_number);
</code>