Updating inherited attributes without __construct()

[Tyson Vanover <tvanover@xxxxxxx>]

I have a class that has a list of valid keys, and an 
array of values.  When a value is added to the array 
it's key is first checked against the list of valid 
keys (this is to prevent injection issues we have been 
having later on in the project).

  class parent{
    private $validkeys = 'title,color,name';
    private $values = array();
  }
That class is inherited by other classes that mostly 
just have an expanded list of valid keys.  I would like 
to be able to update the valid key list without having 
to craft a constructor for child objects.  I would 
rather not give each child class the constructor,

  __construct()
  {
    $this->validkeys.= ',setting,...';
    parent::__construct();
  }

since most child classes have no need of a unique 
constructor.

I thought of defining a protected extended valid keys 
variable in the parent that the child could set that 
the parent constructor would combine with the valid keys.

class parent{
  private $validkeys = 'title,color,name';
  private $values = array();
  protected $extendedkeys;
  __construct(){
    $this->validkeys .= $this->extendedkeys;
  }
}

But the children may become parents for further child 
classes.  And if the sub-child classes use the 
$extendedkeys variable it will overwrite the 
intermediate class's version.

Is there any way to tack on extra additional data by 
the child classes without creating a constructor.  I am 
not wedded to strings as a means, it just needs to be 
able to hold a string value.

Thanks

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