Description:
------------
If a function called by preg_replace_callback throws an 
exception this exception is not propagated up to the 
function which called preg_replace_callback. Instead a PHP 
error is produced. This error contains no information about 
what the problem was or where it occurred.

Reproduce code:
---------------
<?php

function thrower() {
  throw new Exception("Hello");
}

try {
  $b = preg_replace_callback('/./', 'thrower', 'a');
} catch ( Exception $e ) {
  echo 'Yay, exception!';
}

?>

Expected result:
----------------
code to output

Yay, exception!

Actual result:
--------------
Warning: preg_replace_callback(): Unable to call custom 
replacement function on line 13