Sets the default handler function for the XML parser
   parser.
  
parser
handler
       handler is a string containing the name of a
       function that must exist when xml_parse() is called
       for parser.
      
       The function named by handler must accept
       two parameters:
       handler ( resource parser, string data )
       
parserThe first parameter, parser, is a reference to the XML parser calling the handler.
data
           The second parameter, data, contains
           the character data.This may be the XML declaration,
           document type declaration, entities or other data for which
           no other handler exists.
          
If a handler function is set to an empty string, or FALSE, the handler in question is disabled.
Note: Instead of a function name, an array containing an object reference and a method name can also be supplied.