如何在PHP中创建嵌套方法？

For example, I've seen third-party applications that have functions like this:

$db->select('columns')->from('table')->where('condition');

That's just an example. How do you create methods like that?

To accomplish this, each of the methods should return $this, an instance of the class which contains the methods.

class MyClass {

   public function select($x){
      // do something
      return $this;
   }

   public function from($x){
      // do something
      return $this;
   }

   public function where($x){
      // do something
      return $this;
   }

}

inside these methods, you generally perform some kind of modification to the state of the object.

In your example, the methods are simply returning objects. So $db->select() returns an object with a method from(), which returns an object with a method where().