<?php
    echo "<p>simple php calculator</p>";
    echo "<form action='' autocomplete='off' method='GET'>";
       $mydefault="";
        if (isset($_GET["a"])) {
            echo "<input type='text' name='a' value='" . $_GET["a"] . "'>";
            $mydefault=$_GET["operator"];
        }
        else
            echo "<input type='text' name='a'>";
        echo "<select name='operator'>
                    <option>+</option>
                    <option>-</option>
                    <option>*</option>
                    <option>/</option>
                    <option value=" . $mydefault . " selected>" . $mydefault . "</option></select>";
        if (isset($_GET["b"]))
            echo "<input type='text' name='b' value='" . $_GET["b"] . "'>";
        else
            echo "<input type='text' name='b'>";
        echo "=";
        if (isset($_GET["a"])) {
                $a=(int)$_GET["a"];
                $b=(int)$_GET["b"];
                $operator=$_GET["operator"];
                switch ($operator) {
                        case "+":
                            echo $res=$a+$b; 
                            break;
                        case "-":
                            echo $res=$a-$b; 
                            break;
                        case "*":
                            echo $res=$a*$b; 
                            break;
                        case "/":
                            if ($b > 0)
                                echo $res=$a/$b; 
                            break;
                    }
            }
        echo "<br><br>";
        echo "<input type='submit' value='Calculate'>";
    echo "</form>";
?>