Use zval_ptr_dtor() to free variables as soon as they hit refcount of 0.

Note:  You should not be using ZVAL_DELREF() in day to day usage.  Instead,
       you should use zval_ptr_dtor().  Use ZVAL_DELREF() only if you're
       messing with the refcount directly and know what you're doing.
Note #2:  For clarity, if you want to initialize a new zval with a refcount
          of 0, it's best to do that directly, instead of using ZVAL_DELREF
          after allocating the zval...

ext/mysqli/mysqli.c

@@ -70,7 +70,7 @@ void php_free_stmt_bind_buffer(BIND_BUFFER bbuf, int type)
 			}
 		}
 		if (bbuf.vars[i]) {
-			ZVAL_DELREF(bbuf.vars[i]);
+			zval_ptr_dtor(&bbuf.vars[i]);
 		}	
 	}