Make Calculator - When Button clicked the No shows in Textbox.

<?php
$res = 0;
$mathExpression = "((2+2) /4) *5";
eval("\$res = $mathExpression;");
echo $res;
?>

<form method ="POST" action="<?php  ($SERVER["PHP_SELF"]);?>">

<form method ="POST" action="<?php  echo $_SERVER["PHP_SELF"];?>">

<button type="submit" name="number6" value="6" style="width:20px; padding-left:5px; margin-left:2px;">6</button>

<input id='result' type="text" width="50" height="20" name="display" value="<?php  echo $display or $newdisplay or $answer;?>">

<button type="button" name="number6" value="6" onClick="document.getElementById('result').value += this.value">6</button>

<?php

$number1 = (int)($_POST['number1']);
$number2 = (int)($_POST['number2']);
$action  = $_POST['action'];

if(!isset($number1)){
        echo 'Missing first operand!';
}
else if(!isset($number2)){
        echo 'Missing second operand!';
}
else if(!empty($action)){
        if($action == "Add"){
                $result = $number1 + $number2;
        }
        else if($action == "Subtract"){
                $result = $number1 - $number2;
        }
        else if($action == "Multiply"){
                $result = $number1 * $number2;
        }
        else if($action == "Divide"){
                $result = $number2 != 0 ? $number1 / $number2 : "NaN";
        }

        if(isset($result)){
                echo "Your result is: ".$result;
        }
}
?>

<form action="" method="POST">
<input type="number" required="required" name="number1" />
<input type="number" required="required" name="number2" />
<input type="submit" name="action" value="Add" />
<input type="submit" name="action" value="Subtract" />
<input type="submit" name="action" value="Multiply" />
<input type="submit" name="action" value="Divide" />
</form>