title: Conditional validation of fields based on other field value in Symfony2
body: <p>So here is the scenario: I have a radio button group. Based on their value, I should or shouldn't validate other three fields (are they blank, do they contain numbers, etc).</p>
  
  <p>Can I pass all these values to a constraint somehow, and compare them there?</p>
  
  <p>Or a callback directly in the controller is a better way to solve this?</p>
  
  <p>Generally, what is the best practice in this case?</p>