我很难理解为什么下面的代码不起作用。它返回正确的数字，但当它达到-1时，页面应该停止。
日期字段是mysql数据库中的日期。

$date = new DateTime(date("Y-m-d", strtotime($pay_posted1)));
$date->modify('+'.$time_frame1.' months');
$NEW_DATE = $date->format('Y-m-d');

$firstp  = new DateTime(date("Y-m-d")); //CURRENT DATE
$secondp = new DateTime(date("Y-m-d", strtotime($NEW_DATE)));

$diffp = $firstp->diff($secondp);

$DIFFp = $diffp->format('%R%a');
$DIFF_p = $diffp->format('%a');

if ($DIFFp == +0) {
    $PAYMENT_ERROR = "<center><h2><b><font color='#FF0000'>PAYMENT DUE 
    TODAY <a href=\"javascript:void(window.open('payment_history.php', 
     '', 
    'width=500,height=600,top=10,left=40,scrollbars=yes'))\">(View) 
    </a> 
    </font></b></h2></center>";
} elseif($DIFFp <= +10) {
    $PAYMENT_ERROR = "<h2><b><font color='#FF0000'>PAYMENT DUE IN 
    $DIFFp DAY(S)</b></font></h2>";
} elseif ($DIFFp <= -1) {     
    $PAYMENT_ERROR = "<br><br><br><br><h1><b><font 
    color='#ff0000'>PAYMENT IS PAST DUE!! <br>
    PLEASE FOLLOW THIS <a 
    href='http://wawoffice.net/contact.php'>LINK</a></font></h1>";
   exit();
} else {
   $PAYMENT_ERROR = "";
}

谢谢