Description:
------------
Calling print_r(debug_backtrace()) from a method of a class that inherits from 
mysqli overwrites the error member variable. Calling debug_backtrace() on its own 
doesn't seem to cause any problems, but when combined with print_r, this problem 
results.

Test script:
---------------
<?php
class Db extends mysqli {
	function f() {
		$this->query("select");
		var_dump($this->error);
		print_r(debug_backtrace(), true);
		var_dump($this->error);
	}
}

$db = new Db("host", "user", "password", "database");
$db->f();

Expected result:
----------------
string(146) "You have an error in your SQL syntax; check the manual that 
corresponds to your MySQL server version for the right syntax to use near '' at 
line 1"
string(146) "You have an error in your SQL syntax; check the manual that 
corresponds to your MySQL server version for the right syntax to use near '' at 
line 1"

Actual result:
--------------
string(146) "You have an error in your SQL syntax; check the manual that 
corresponds to your MySQL server version for the right syntax to use near '' at 
line 1"
string(0) ""