PHP Self-referencing script

I am trying to embed a self-referencing PHP script inside an HTML form with following code:

Undefined index: conv

<form action = "<?php $_SERVER['PHP_SELF'] ?>" method = "post">
    <input type = "number" id = "temp2" name = "temperature2" placeholder = "28">
    <label for = "temp2"> degrees </label>

    <select>
        <option name = "conv" value = "f"> Fahrenheit </option> 
        <option name = "conv" value = "c"> Celsius </option>
    </select>   

    <input type = "submit" value = "equals">

    <?php
        $type = $_POST["conv"];
        $tmp = $_POST["temperature2"];
        if ($type == "f") {
            $newTmp = (9/5 * $tmp) + 32;
            echo $newTmp . " degrees Celsius.";
        }
        elseif ($type == "c") {
            $newTmp = (5 * ($tmp - 32)) / 9;
            echo $newTmp . " degrees Fahrenheit."; 
        }
    ?>

</form>

And I am getting this messages: Notice: Undefined index: conv Notice: Undefined index: temperature2

Everything worked fine when the php script was in another file. Anyone knows what am I doing wrong?