calc

<!DOCTYPE html>
<html lang="de">
<head>
<meta charset="utf-8">
<title>dEn Cliqulatoz</title>
<h1>THe KLaCULATIOR</h1>
</head>
<body bgcolor="#FF00FF" background="http://bestanimations.com/Animals/Birds/Parrots/animated-parrot-gif-2.gif">

    <form action="" method="Post">
<p>z1 Value:<br/>
        <input type="text" name="z1" value="<?PHP if(!empty($_POST['z1']))echo $_POST['z1']?>"><br><br>
<p>z2 Value:<br/>
        <input type="text" name="z2" value="<?PHP if(!empty($_POST['z2']))echo $_POST['z2']?>">
        <br><br>

<button type="submit" name="group1" id="add" value="add">+</button>
<button type="submit" name="group1" id="subtract" value="subtract">-</button>
<button type="submit" name="group1" id="times" value="times">*</button>
<button type="submit" name="group1" id="divide" value="divide">/</button>
<p>
<button type="submit" name="group1" id="force" value="force">^</button>
<button type="submit" name="group1" id="sqrt" value="sqrt">²√</button>
<!-- <button type="submit" name="group1" id="xrt" value="divide">/</button>
 <button type="submit" name="group1" id="divide" value="divide">/</button> -->

    </form>


<p>
answer:
<p>
<h1>
<?php
/*
if (!empty($_POST['z1'])){
$z1 = $_POST['z1'];
$z2 = $_POST['z2'];
else if($_POST['group1'] == 'sqrt') {
echo sqrt($z1);
}
else if(!empty($_POST['z1']) && !empty($_POST['z2'])){

$z1 = $_POST['z1'];
$z2 = $_POST['z2'];
if($_POST['group1'] == 'add') {
echo $z1 + $z2;
}
else if($_POST['group1'] == 'subtract') {
echo $z1 - $z2;
}
else if($_POST['group1'] == 'times') {
echo $z1 * $z2;
}
else if($_POST['group1'] == 'divide') {
echo $z1 / $z2;
}
else if($_POST['group1'] == 'force') {
echo pow($z1 , $z2);
}
}
}
else{}
*/
if(!empty($_POST['z1']) && !empty($_POST['z2'])){
switch($_POST["group1"]){
    case "add":
        echo ($z1 + $z2);
        break;
    case "subtract":
        echo ($z1 - $z2);
        break;
    case "times":
        echo ($z1 * $z2);
        break;
    case "divide":
        echo ($z1 / $z2);
        break;
    case "force":
        echo (pow($z1, $z2));
        break;
    case "sqrt":
        echo (sqrt($z1));
        break;
//  case(){$z1 $z2}
//  case(){$z1 $z2}

}}
else{}
?>

</h1>
</form>
</body>
</html>