Description:
------------
If you whrite a string like "http://..." or 'http://...' into the xpath_register_ns function there will occur a problem with a stringlength between 16 and 23 characters and also between 40 and 47 characters.
Example:
$ctx->xpath_register_ns ("pre", 'http://www.xyz.de/');

The result is the same when the registered uri differs from the uri in the XML-file: 
There is no result object if evaluate an xpath with the registered namespace (like "/pre:foo").

If you define the uri in a variable (i.e. $uri) and write the xpath_register_ns() function with all is fine.

Reproduce code:
---------------
<?PHP
$s = '<?xml version="1.0" ?>';
$s .= '<test:foo xmlns:test="http://www.xyz.de/abc.xyz">';
$s .= '<test:bar>abc';
$s .= '</test:bar>';
$s .= '</test:foo>';
// namespace uri as string
$doc = domxml_open_mem($s); 
$ctx_p = xpath_new_context($doc);
$reg_succ_p = $ctx_p->xpath_register_ns ("test", 'http://www.xyz.de/abc.xyz');
$p = xpath_eval($ctx_p, '/test:foo/test:bar');

$uri="http://www.xyz.de/abc.xyz"; //namespace uri in $uri
$ctx_q = xpath_new_context($doc);
$reg_succ_q = $ctx_q->xpath_register_ns("test", $uri);
$q = xpath_eval($ctx_q, '/test:foo/test:bar');

echo '<PRE><br>$p: ';    print_r($p);
echo '<br>$q: ';    print_r($q);
echo "</PRE>";
?>

Expected result:
----------------
Both xpath_eval commands shood produce the same result:

The expected output is:

$p: XPathObject Object
(
    [type] => 1
    [nodeset] => Array
        (
            [0] => domelement Object
                (
                    [type] => 1
                    [tagname] => bar
                    [0] => 4
                    [1] => 135761016
                )

        )

)
$q: XPathObject Object
(
    [type] => 1
    [nodeset] => Array
        (
            [0] => domelement Object
                (
                    [type] => 1
                    [tagname] => bar
                    [0] => 4
                    [1] => 135761016
                )

        )

)


Actual result:
--------------
Only the second xpath evaluation where the registered uri is given as variable ($uri) is successful.
$p: 
$q: XPathObject Object
(
    [type] => 1
    [nodeset] => Array
        (
            [0] => domelement Object
                (
                    [type] => 1
                    [tagname] => bar
                    [0] => 5
                    [1] => 136278112
                )

        )

)