PHP查询结果=空，然后div隐藏

Question

I have a calculator function that displays the result when a user enters a query in a div (calcanswer) but often the query is not a calculation request, so the result is "query=" in which query represents the user input, and then nothing behind the = sign. I am wondering whether it is possible to implement a function that hides the div when this happens (ie there is no calculation).

PHP:

<?php
$a=$_GET['q'];
//$a="1/2";
$add = stripos($a, '+') !== false;
$sub = stripos($a, '-') !== false;
$mul = stripos($a, '*') !== false;
$div = stripos($a, '/') !== false;
if($add){
    $b=explode("+",$a);
    $n1=(float)$b[0];
    $n2=(float)$b[1];
    $n3=$n1+$n2;
}else if($sub){
    $b=explode("-",$a);
    $n1=(float)$b[0];
    $n2=(float)$b[1];
    $n3=$n1-$n2;
} else if($mul){
    $b=explode("*",$a);
    $n1=(float)$b[0];
    $n2=(float)$b[1];
    $n3=$n1*$n2;
} else if($div){
    $b=explode("/",$a);
    $n1=(float)$b[0];
    $n2=(float)$b[1];
    $n3=$n1/$n2;
}
?>

HTML:

<div class="calcanswer"><center>
  <h4 class="card-title pb-3 mbr-fonts-style display-7">
  <?= $a."=".$n3 ?>
</h4></center></div>

Answer 1

I have been able to fix it by doing this:

<?php
        if(isset($n3)) {
        ?>

        <div class="calcanswer"><center>
          <h4 class="card-title pb-3 mbr-fonts-style display-7">
           <?= $a."=".$n3 ?>
        </h4></center></div>

        <?php } ?>

I want to thank @Tim Hinz for helping me in such a short amount of time & Jim Grant for taking time out of their day to help me.

Answer 2

Just do this:

<?php 
if(isset($_GET["query"])) {
?>

<div class="calcanswer"><center>
  <h4 class="card-title pb-3 mbr-fonts-style display-7">
   <?= $a."=".$n3 ?>
</h4></center></div>

<?php } ?>