Concern about changing zval's passed by reference

Hello I am using the macros ZVAL_BOOL, ZVAL_LONG, ZVAL_DOUBLE and ZVAL_STRING to change the value of a zval passed by reference. I noticed that these macros do not free the original value if it's a string, array, object or resource prior to the change. This causes a possible memory leak. Is this a normal practice? And, if not, what is the accepted practice for changing zval's passed by reference? -- bob