与条纹工作，无法找出这些语句上的错误

我试图根据人们使用Stripe的货币收取不同的费用。我认为if语句中的另一个条件是足够的，但无论我选择美元还是欧元，它似乎都是假的，并执行else语句。

 //** Now to provide different plans for differnet currencies. **'' 
 if(isset($_POST['subscription_purchase']) && 'currency' == 'euro')
 {
   echo 'Euro is working';
 }
 elseif(isset($_POST['subscription_purchase']) && 'currency' == 'dollar')
 {
    echo 'Dollars are working'; 
 }
 else 
 {
    echo 'This shouldnt happen';
 }
 The form which this acts on is the following:
 <form action="charge.php" method="post">
    <h3>Sign up to get a new quote every day! Just &euro;400 a month! <small>(Subscription)</small></h3>
    <input type="hidden" name="subscription_purchase" value="subscription_purchase" />
    <input type="text" name="email" placeholder="Email Address" />
    <input type="password" name="password" placeholder="Password" />
    <select name="currency" id="currency">
        <option value="euro">Euro</option>
        <option value="dollar">USD</option>
        <option value="rand">RND</option>
        <option value="pounds">GBP</option>
    </select>
    <script 
        src="https://button.stripe.com/v1/button.js" class="stripe-button"
        data-key="<?php echo $stripe['publishable_key']; ?>"
        data-amount=40000
        data-description="Wilde monthly subscription"
        data-label="Checkout">
    </script>
 </form>