PHP Manual: xml_set_processing_instruction

Description

bool xml_set_processing_instruction_handler ( resource parser, callback handler )

Sets the processing instruction (PI) handler function for the XML parser parser. handler is a string containing the name of a function that must exist when xml_parse() is called for parser.

A processing instruction has the following format:

<?target 
       data?>

You can put PHP code into such a tag, but be aware of one limitation: in an XML PI, the PI end tag (?>) can not be quoted, so this character sequence should not appear in the PHP code you embed with PIs in XML documents. If it does, the rest of the PHP code, as well as the "real" PI end tag, will be treated as character data.

The function named by handler must accept three parameters: handler ( resource parser, string target, string data )

parser: The first parameter, parser, is a reference to the XML parser calling the handler.
target: The second parameter, target, contains the PI target.
data: The third parameter, data, contains the PI data.

If a handler function is set to an empty string, or FALSE, the handler in question is disabled.

TRUE is returned if the handler is set up, FALSE if parser is not a parser.

Note: Instead of a function name, an array containing an object reference and a method name can also be supplied.