Description:
------------
This is a hopefully simple request to add an extra visibility type into classes to 
allow for paramaters that can be read outside the class like with public but can 
only be written too inside as with protected. I've put a sample of how I think 
this should work below to hopefully give you a better idea of this.

Test script:
---------------
class Test
{
  readonly $param_ro;

  public funcion test()
  {
    $this->param_ro = 'test-ro'; // Works
  }
}
$test = new Test();

echo $test->param_ro; // Works

$this->param_ro = 'test'; // Fails