PHP, what does that mean? $ctrl $action (); How to handle

PHP, what does that mean? $ctrl $action ();
$ctrl- $action ();

$ctrl is an object that accesses what is this thing?
--------------------------------------------------------------
The Access property is this $ctrl->action;
The Access class method is this $ctrl->action ();

------Solution--------------------
$ctrl $action (); It is also called a method name, except that the name of the method is replaced with a variable $action.
------Solution--------------------
function fn () {
Echo 1;
}

$FN = ' fn ';
$FN ();

It's just a variable of the function or object method name to run in the same way
However, it is recommended to check with Function_exist and method_exist before use

