title: Symfony2 How to add required form field dynamically
body: <p>In Form-Builder, I have a choice of fields for a <code>Credit Card</code> or <code>Bill Payment Type</code>.</p>
  
  <p>Depending on what is selected, other form fields will be loaded <code>i.e. CC-Number or Bank Account</code>.</p>
  
  <p>I need the CC-Number input to be a required field, if Credit Card is selected. How can I achieve this?</p>