title: Find all letters not used in a string
body: <p>I want to be able to do something likes this:</p>
  
  <pre><code>$str="abc";
  echo findNot($str); //will echo "defghijklomnopqrstuvwxyz"
  $str2="happy birthday";
  echo findNot($str2); //will echo "cfgjklmnoqsuvwxz"
  </code></pre>
  
  <p>Basically, it would find all letters not represented in the string and return them in an array or string.</p>
  
  <p>I could do this easily with a <code>foreach</code> and arrays of characters, but I was wondering if anyone had a more elegant solution.</p>