Description:
------------
Passing a SimpleXML string object to most builtin string functions causes a memory leak.

Reproduce code:
---------------
#!/usr/local/bin/php -d memory_limit=128M
<?
    $xml = '<?xml version="1.0" encoding="utf-8"?>';
    $x = simplexml_load_string($xml . "<q><x>foo</x></q>");

    echo "explicit conversion\n";
    for ($i = 0; $i < 1000000; $i++)
        md5(strval($x->x));

    echo "no conversion\n";
    for ($i = 0; $i < 1000000; $i++)
        md5($x->x);

    echo "done\n";
?>


Expected result:
----------------
$ ./crash.php
explicit conversion
no conversion
done


Actual result:
--------------
$ ./crash.php
explicit conversion
no conversion

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 40 bytes) in /tmp/crash.php on line 12