Custom validation - credit card

<?php
$feedback = "";
$success_message = "Thank you for your donation!";
$error_message = "* There was an error with your card. Please try again.";

$card_type = "";
$card_num = "";
$donation_amount = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $card_type = $_POST["credit"];
    $card_num = $_POST["card-num"];
    $donation_amount = $_POST["amount"];

  if (strlen($card_num) > 100){  // if card no. has more than 100 characters, we won't use regex checking
    $feedback = $error_message;
  } else {  // we will do regex checking here
    if ($card_type === "mastercard") {
      if (preg_match("/5[1-5][0-9]{14}/", $card_num)) {
    $feedback = $success_message;
      } else {
        $feedback = $error_message;
      }
  } elseif ($card_type === "visa"){
    if (preg_match("/4[0-9]{12}([0-9]{3})?([0-9]{3})?/",$card_num)){
      $feedback = $success_message;
    } else {
      $feedback = $error_message;
    }
  }
  }
}

?>
<form action="" method="POST">
  <h1>Make a donation</h1>
    <label for="amount">Donation amount?</label>
      <input type="number" name="amount" value="<?= $donation_amount;?>">
      <br><br>
    <label for="credit">Credit card type?</label>
      <select name="credit" value="<?= $card_type;?>">
        <option value="mastercard">Mastercard</option>
        <option value="visa">Visa</option>
      </select>
      <br><br>
      <label for="card-num">Card number?</label>
      <input type="number" name="card-num" value="<?= $card_num;?>">
      <br><br>
      <input type="submit" value="Submit">
</form>
<span class="feedback"><?= $feedback;?></span>