When I was reading an open source code recently, I noticed that others set the second parameter to true when using the is_callable function. I wanted to know the specific meaning of the second parameter, so I read the manual later. There is also a third parameter, but I still don’t understand what the manual means. I hope friends who know more about it can give me some advice: 
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
http://php.net/manual/zh/func... There are examples in the manual.