Description:
------------
When passing an object with a __toString() method to a 
function that uses zend_parse_parameters() with an "s" 
value, PHP does not correctly coerce the object into a 
string.

This occurs for both extensions (like SimpleXML) and 
user-defined PHP classes.

Reproduce code:
---------------
class foo {
    function __toString() {
        return "foo";
    }
}

$foo = new foo;
print strrev($foo) ."\n";
print htmlspecialchars($foo) ."\n";

Alternatively:

$xml = '<?xml version="1.0" ?><root><element>text</element></root>';
$s = simplexml_load_string($xml);
print strrev($s->element) ."\n";
print htmlspecialchars($s->element) ."\n";

Expected result:
----------------
oof
foo

Actual result:
--------------
PHP Warning:  htmlspecialchars() expects parameter 1 to 
be string, object given in foo.php on line 10
oof

strrev() uses convert_to_string_ex(), so it works fine. 
htmlspecialchars() uses zend_parse_parms(), so there's a 
warning.