Why is the $ _ GET [] array a method?

Why is the $ _ GET [] array a method of it here? the object calling method in it is not the index () method, but the method --> $ () why?

Reply to discussion (solution)

$ _ GET ['A'] is a parameter name on the hyperlink. $ a = is not $ _ GET. you can view the name of $.

My problem is:
Why not write $ controller-> index () here?

It is written as $ contrlooer-> $ ()

Jian Zun: happy
There's no problem writing this.

[Jian Huang] Pandora 14:14:37
Why not write $ controller-> index () here?
14:14:50
[Jian Huang] Pandora 14:14:50
It is written as $ contrlooer-> $ ()
[Jianzun] July 22, 14:15:13
$ Controller-> index () cannot be written to death.
14:16:52
[Jianzun] July 22, 14:16:52
In this case, a in the url can only be equal to the index method ..
[Jianzun] July 22, 14:17:08
A controller must be more than an index method.

To put it bluntly, here is the reference to pass the value