Description:
------------
The following (ill-advised but not uncommon) code...

   preg_replace('/'.$a.'/i', $b, $c)

will eval $b if $a contains "/e\0".

Good fix: turn null bytes into backslash-zero before passing them to PCRE.

Easier fix: check that 'pp' is actually at the end of the string and not just at 
some random null byte after the modifier loop in pcre_get_compiled_regex_cache, 
and error out if not.

Test script:
---------------
 $word = "die('Lalala'); 1||1/e\0";

// Slightly contrived example, but let's pretend we're
// case-normalizing a user-provided word, and forgot to
// preg_quote

$text = preg_replace('/'.$word.'/i', $word, $text);


Expected result:
----------------
"preg_replace(): Unknown modifier '\0' in ..."

(or some better error message)

Actual result:
--------------
Code execution.