In PHP-5.6 and below each argument passed to user function was copies on VM stack twice.
Now we always have ZEND_INIT_FCALL (or simular) opcode that pushes "call frame" on top of VM stack.
"Call frame" is actually the same zend_execute_data structure.
All the following ZEND_SEND instructions push arguments on top of the stack in a way that they directly comes into corresponding CV variables of the called frame. Extra arguments are copied at the end of stack frame (after all CV and TMP variables) on function enterance.
There are two minor incompatibilities:
1) It's not allowed to decalre functions redefining arguments e.g. "function foo($a,$a) {}".
2) func_get_arg() and func_get args() return the current value of argument and not the original value that was sent.
It should be accessed using Z_CACHE_SLOT() macro.
zend_literal structure is removed.
API functions that accepted pointer to zend_literal now accept pointer to zval or cache_slot directly.
Calls of such functiond that now accept cache_slot need to be changed to pass -1 instead of NULL.
Dmitry Stogov