function drawMe()
{
    if( 2 == func_num_args() ) {
        doStuff( func_get_arg(0) , func_get_arg(1) ) ;
    } else {
        doStuff( $defaultX , $defaultY ) ;
    }
}

private vars/methods would be good... :)

I completely disagree with the concept of dynamic binding.
Calling different functions based on the number of arguments
is spaghetti code waiting to happen.  PHP supports default
values for arguments which makes them optional.  Your 
would be much cleaner if written with defaulted x and y 
arguments which you then check and if you have the default
value for x and/or y you would calculate the center value.
Or alternatively use func_num_args to see how many args were
passed.  To me it is cleaner to collect the functionality for a function in a single function instead of splitting it 
out over multiple functions.  Imagine trying to debug some
code and you end up looking at the wrong function just
because you counted 17 arguments instead of 18.

As for the public/private issue.  Sure, from an OO purity
perspective it would be nice.  But it is functionally irrelevant.  ie. your code would not work any better or
worse by having the public/private differentiation.

I disagree with your comment on how dynamic bindings are used.  The whole purpose of dynamic binding is give a function whose purpose could be asked for in many different ways yet is asking to do the same thing the ability to hide how it works from the programmer using it.  In C++, there is a function of exp, which could be sent four different methods:

int exp (int base, int exponent)
double exp (int base, double exponent)
double exp (double base, int exponent)
double exp (double base, double exponent)

If anything, it makes the programmers job easier to utilize this function because they only have to remember ONE function, not four different ones accomplishing the same task.

Admittedly, PHP doesn't have data types in the traditional sense, however it should have Dynamic Binding because one of the most fundamental rules of Object Oriented Programming is the idea of hiding the how and just utilizing the functions and classes.

The same argument can be made for the public/private distinction.  Adding this feature is an excellent way to make sure that programmers do not mess with the logical code that you worked so hard on.  Also, it forces programmers to use the functions of the class, as the orginal programmer percieved it.

I want to see PHP become the best Web Programming Language out there, and proper OOD techniques is fundamental to getting there.

multiple dispatch is not going to happen. ZE2 has private member values.