Description:
------------
---
From manual page: http://www.php.net/class.domxpath
---

in some cases the return value of DOMXPath::evaluate could be unfavorable. 
For example what if a valid xpath expression like 
'false()' or 'count(...)=0' evaluates to false and you want to catch 
e.g. malformed xpath expressions which return false as well? 
how to distinguish those scenarios? 

therefor it would be useful to have a function to 
 - check the xpath expression first or
 - having some kind of unique value returned 
   in case the evaluation failed 



Test script:
---------------
<?php
  
  $d = new DOMDocument();
  $xs = '<?xml version="1.0" ?><root><foo /></root>';
  $d->loadXML($xs);
  $x = new DOMXPath($d);

  // simple eval
  $eval = $x->evaluate('false()');
  if(!$eval) { echo "BOOBOO"; }

  // complex but valid eval
  $eval = $x->evaluate('count(//foo)=0');
  if(!$eval) { echo "BOOBOO"; }

  // bad eval
  $eval = $x->evaluate(')');
  if(!$eval) { echo "BOOBOO"; }

?>