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