There are several options:

 1) When you come across an identifier you haven't seen before - see if it is already 
    declared as a symbol in the containing class and assume it refer's to that if it
    is.  This has the problem that it is not compatible with previous versions of PHP.

 2) Make it compulsary to declare local variables, then use the normal strategy, which
    is to treat as a local variable if one is declared, else treat as a class variable, 
    else treat as a global.  This is also not backwards compatible.

 3) Introduce a new keyword - similar in concept to global - that declares an
    identifier as refering to a member: the keyword `member' would be a good 
    candidate - this would also be backwards compatible.  The disadvantage of this
    scheme is that it'd be a pain (but probably not as much as $this).

 4) Introduce some sort of pragma indicating to the compiler that this function is
    aware that you are using strategy 1) or 2).  For example you could delcare
    functions:

       member function get_property($x) { ... }

    or come up with a replacement new keyword:

       method get_property($x) { ... }

    that has this different property - its better to declare variables anyway :)

I think option 4) is most desirable, and option 3) is the easiest to do.  I doubt you guys would even consider breaking compatibility.  Option 4 also gives you the option of introducing other OO things such as public/private/package if you like that sort of thing.  And you can still support all the legacy code.

*If* PHP will be extended the way to improve OO and force
the OO coder who wants to use this to take care of more
things at compile time, then this will possibly included...

This will not be changed.