php - NULL radio button value -

on line 13 of code, getting null $nan value. if replace $_post['amount'] static integer, code post correct value. trying add little functionality, can't data pass???

http://i40.tinypic.com/34gxauw.png

<div id="container"> <?php     require_once('stripe-php/lib/stripe.php');     $stripe = array(     'secret_key' => 'sk_07c5ukidqxydgfqikb8f7txqggyqt',     'publishable_key' => 'pk_07c58bhnzufyqr9auozd6scliwanw'     );     stripe::setapikey($stripe['secret_key']);      if ($_post) {         $charge = stripe_charge::create(array(             'card' => $_post['stripetoken'],             'amount' => $_post['amount'],             'currency' => 'usd'         ));         var_dump($_post['amount']);         $quotes = array(             "thank purchase!",             "enjoy experience with us!"         );          echo "<h1>here's quote!</h1>";         echo "<h2>".$quotes[array_rand($quotes)]."</h2>";     }     else { ?>      <h2>tpc holdings</h2>     <h3>select campaign package!!</h3>      <form>         <fieldset>             <legend>select package</legend>             <p>                 <label>select package</label>                 <input type = "radio"                         name = "amount[]"                         id = "sizesmall"                         value = "50"                         checked = "checked" />                 <label = "sizesmall">$50.00</label>                  <input type = "radio"                         name = "amount[]"                         id = "sizemed"                         value = "75" />                 <label = "sizemed">$75.00</label>                  <input type = "radio"                         name = "amount[]"                         id = "sizelarge"                         value = "large" />                 <label = "sizelarge">$100.00</label>            </p>        </fieldset>     </form>      <form action="paybill.php" method="post">         <script src="https://button.stripe.com/v1/button.js"              class="stripe-button"              data-key="<?php echo $stripe['publishable_key']; ?>"              data-amount="<?php echo $d_charge; ?>"              data-description="tpc purchase"              data-label="buy"></script>     </form>  <?php   } ?>  </div><!-- #container --> 

don't make amount field array user select one these.

try this,

<p>     <label>select package</label>     <input type = "radio"         name = "amount"         id = "sizesmall"         value = "50"         checked = "checked" />     <label = "sizesmall">$50.00</label>      <input type = "radio"         name = "amount"         id = "sizemed"         value = "75" />     <label = "sizemed">$75.00</label>      <input type = "radio"         name = "amount"         id = "sizelarge"         value = "large" />     <label = "sizelarge">$100.00</label> </p>