I'm trying to learn PHP and I wish to know if there are some function equivalence constructs:

In JS I can do:

var t = function() {};
t();

myObj.DoStuff = function() {} // here I add a method at run-time
myObj.DoStuff();
myObj["DoStuff"]();

var j =  myObj.DoStuff;
j();

and so other things with function evaluation.

In Js objects are associative arrays so every properties is accessible with the subscript notation...

Can I add a method at run-time to a class template so next its object instances can have it?

In JS I can do that via a prototype function property.