Description:
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I believe having the ability to define read & write permissions on class members would help with certain problems encountered when currently trying to implement similar features using __get() and __set().

I would like to propose that three "flags" be defined and would be used with the public, protected and private declarations.  The flags would be ro (read-only), wo (write-only) and rw (read-write; for completeness).  They would be prepended (or appended) to a visibility declarations, for example: ro_public, rw_protected or wo_private.

These three flags would allow more control over member access than attempting to use __get() and/or __set() which, to achieve similar results, could require many lines of additional logic and code.  As it is currently in PHP 5.2.x, due to the implementation of trigger_error() (in that you cannot perfectly mimic an internal E_ERROR), it is not possible to transparently implement features such as what I am proposing here.

Setting the various flags on the different visibility declarations would have differing results.  I will (attempt) to explain how these flags would function depending on visibility.

* Setting ro/wo on a public declaration would mean that public access can only read/write to that member; access by direct and extended classes of that member would be unrestricted.
* Setting ro/wo on a protected declaration would mean that extended classes can only read/write to that member; public access is still prohibited and direct class access is still unrestricted.
* Setting ro/wo on a private declaration would mean that direct class access is read/write only; public and extended class access is still prohibited.

Note: from the examples, you can see why the "rw" flag isn't required - I mentioned it simply for completeness if it would make sense to allow people to define it, otherwise it can be omitted since it is simply the "default" access permission.