Doing something with an each opcode as zend_execute() handles it

Just before zend_execute()/execute() handles each opcode in its big switch() statement, I'd like to be able to call a function and pass it the opcode (or other information from the opline. Is the best approach to reset zend_execute to a new function in the PHP_MINIT_FUNCTION() (and restore the saved value in PHP_MSHUTDOWN_FUNCTION)? This new function would mostly be a copy of execute() in zend_execute.c with my addtional function call tossed in. What's the difference between zend_execute() and zend_execute_internal()? Thanks, David