Disable validation for heredoc and nowdoc in PHP code.

[andkirby]

Validation for heredoc and nowdoc is redundant.

        $xml1 = <<<XML
<?xml version="1.0" encoding="UTF-8"?>
<some_name>
    <section attr10="val1" attr20="val2">
        <collection_node>
            <child_node1 />
        </collection_node>
    </section>
    <value_node>12</value_node>
    <unpair_tag/>
</some_name>
XML;