title: PHP - How to check if an input filter is valid?
body: <p>I've got a function that takes a filter constant for <code>filter_input</code> as an optional parameter. Is it possible to make sure the value of the string is a valid PHP filter constant, perhaps with a built in PHP function?</p>
  
  <p>I know one way is to make my own array of all filter constants and check against that, but I was hoping for an easier or better way.</p>