Add a hook for a processing instruction.
Add a hook for a processing instruction. Processing instructions are tags starting with . You are forbidden to use xml in any combination of case (xml, xMl, Xml etc.), but other basic tags such as xml-stylesheet are possible. If you try registering "xml" in any case combination, this function returns an error.
The hook will get a pointer to the relevant xmltag instruction (without the question mark) as its first argument.