I was going to see if the forum could help me with this, but I managed to figure it out. Criticism invited.
PHP Code:
<?php
    $_POST
['CCMonth']="10"// change as needed to test this code
    
$_POST['CCYear']="10"// change as needed to test this code
    
extract($_POST);
    print 
"Input for CC Exp. is ".$CCMonth."/?/20".$CCYear."<br />";
        
    
$Today=date('m/d/Y');
    print 
"Today is ".$Today."<br />";
    
$today=strtotime($Today);
    
    if ((
$CCMonth=="01")||($CCMonth=="03")||($CCMonth=="05")||($CCMonth=="07")||($CCMonth=="08")||($CCMonth=="10")||($CCMonth=="12")){
        
$strDate date("m/d/20y"mktime(000$CCMonth31$CCYear));
    };

    if ((
$CCMonth=="04")||($CCMonth=="06")||($CCMonth=="09")||($CCMonth=="11")){
        
$strDate date("m/d/20y"mktime(000$CCMonth30$CCYear));
    };

    if ((
$CCMonth=="02")&&(date('L'strtotime("20".$CCYear."-01-01"))==0)){
        
$strDate date("m/d/20y"mktime(000$CCMonth28$CCYear));
    };
    
    if ((
$CCMonth=="02")&&(date('L'strtotime("20".$CCYear."-01-01"))!=0)){
        
$strDate date("m/d/20y"mktime(000$CCMonth29$CCYear));
    };

    print 
"CC Exp. interpreted as ".$strDate."<br />";
    
$strDate strtotime($strDate);
    print 
"CC Exp. PHP strtotime is ".$strDate."<br />";
    print 
"Today&rsquo;s date PHP strtotime is ".$today.".";
    
    if (
$strDate $today){
            
$errors=1;
            
$error.="<li>Credit card has expired </li>";
    }
    
    if (
$errors=1){
    print 
"<ul>".$error."</ul>";
    }
?>
Remove the various “print” commands, which are there only for testing, & adapt to suit your needs.