Make PHP Calculator Store and Display History

I am new to PHP and I am trying to create a simple calculator that also displays history (past calculations) in a div on the page.

The first field enters something like "4+" and "3" in the second and it displays "4 + 3 = 7" in a div called "results". However, I want to show the entire history of results there, so when I do a new calculation, the div result will be displayed in both calculations / results. What's the best way to do this in PHP? Can you choose DOMNode::appendChild

?

Here is my code:

<div id="form">
    <form action="" method="post">
        <input type ="text" name="firstNumber">
        <input type ="text" name="secondNumber">
        <input type ="submit" style="display:none">
    </form>
</div>

Result:

<?php 

    $number1 = $_POST["firstNumber"];
    $number2 = (int)$_POST["secondNumber"];

    @$operator = substr($number1, -1);


switch($operator){
    case '+':
        $result = (int)$number1+$number2;
        echo $result;
        break;
     case '-':
        $result = (int)$number1-$number2;
        echo $result;
        break;
    case '*':
        $result = (int)$number1*$number2;
        echo $result;
        break;
    case '/':
        $result = (int)$number1/$number2;
        echo $result;
        break;
}

?>


<div id="results">
    <?php echo substr($number1,0, -1);
        echo $operator; 
        echo $number2; ?>
 <br>=
    <?php echo $result; ?>
</div>

      

        
        
        
      

    

Thanks for helping!