Checking if a link within a form is clicked with php

Question

I have:

  <form action="createPoll.php" method="POST">
    <!-stuff->

        <h3>Number of Options</h3>
        <input type="text" id="numOfOptions" name="numOfOptions" value=""><br />
        <a href="#?clicked=true" id="filldetails" onclick="addFields()">Select</a>
            <div id="container"> </div>
        <input type="submit" value="Submit">
</form>

I want to check whether or not the "Select" is clicked when user clicks submit. I added the "?clicked=true" in the "a href" with the hopes that I could use something like

 if($_GET['clicked'] != "true"){
    some kind of error

But it gives me the error "Undefined index"

Anyway I can do this?