Description:
------------
+= operand empties variable, see code for more details.

Reproduce code:
---------------
<?php
//Running PHP Version 5.2.6 on Windows XP
$foo = '_123';
$bar = '';
$foo += $bar;

echo $foo;
?>

Expected result:
----------------
_123

Actual result:
--------------
<nothing>
Checked with a debugger, $foo += $bar; makes $foo = null;