<?php

   function showMe( $result, $fcnTried, $rgb ){

     if  (   $result  ) {
        echo "<pre style='color:$rgb'><b>$fcnTried(\$_POST['creditCardNumber']) returned TRUE</b> \n";
        print_r($_POST);
        echo "</pre>";

     } else
        echo "<pre style='color:$rgb'><b>$fcnTried(\$_POST['creditCardNumber']) returned FALSE</b> \n";

   }//end showMe

   $result_isset   =   isset($_POST['creditCardNumber'])   ;
   $result_empty   =   empty($_POST['creditCardNumber'])   ;
   $result_is_null = is_null($_POST['creditCardNumber'])   ;

   showMe( $result_isset  ,  "isset"  ,  "maroon" );
   showMe( $result_empty  ,  "empty"  ,  "blue"   );
   showMe( $result_is_null,  "is_null",  "green"  );

?>

<!-- ...................................................................................... -->

<pre>
<form method=post action="AdultForm.php">
 <span class=required>Card Number   <input class=required name='creditCardNumber'></span>
 Security Code <input name='digit3code'>
 First Name    <input name='firstname'>
 Last Name     <input name='lastname'>
 <button> Do Something!</button>
</form>
</pre>

</pre>




<style>
   .required{color:red}
</style>
   

isset($_POST['creditCardNumber']) returned FALSE 
empty($_POST['creditCardNumber']) returned TRUE 
Array
(
)
is_null($_POST['creditCardNumber']) returned TRUE 
Array
(
)




 Card Number   
 Security Code 
 First Name    
 Last Name     
  Do Something!