title: Magento - Dynamically change discount cost in cart
body: <p>In my magento store how can i go about changing the discount total in my shopping cart dynamically? </p>
  
  <p>I'm able to access and get the current discount with this code:</p>
  
  <pre><code>&lt;?php
  $quote = Mage::getSingleton('checkout/cart')-&gt;getQuote();
  $totals =  $quote-&gt;getTotals(); 
  $discount = $totals["discount"]-&gt;getValue();
  ?&gt;
  </code></pre>
  
  <p>I have a button in my shopping cart that when pressed should add an extra £5 onto the discount value, updating the total cost etc at the same time.</p>