php
is_null:
use === intstead of is_null
isset($var) and is_null($var) 是相反的；is_null只有在变量是NULL或者变量声明了但是没有赋值时返回true
is_callable :
if the application could call a method on the object, from the current scope,
then use is_callable
method_exists :
only check this method exist in specified class or not, it will not check it
can run from current scope