extending DateInterval, unknown property error

Question

I'm using PHP's new(ish) Date classes for a calendar/scheduler I'm developing. I am trying to extend DateInterval so that I limit the interval to particular sizes like 1 year, 1 month, 1 week or 1 day. The extended class would be used to help generate the calendar view.

I'm getting the PHP error: Unknown property (days) when I run the following method.

private function adjust(){

     //to zero all the DateInterval properties except the one I want
    $adjustments = array ('y'=>0, 'm'=>0, 'd'=>0, 
                           'h'=>0, 'i'=>0, 's'=>0, 
                           'invert'=>0, 'days'=>0); 


    if($this->y>=1 || $this->days>180){
     $adjustments['y']=1; //1 year
    } else if ($this->m>6){
     $adjustments['y']=1; //1 year
    } else if ($this->m>=1){
     $adjustments['m']=$this->m; //multiple months less than 6
    } else if ($this->d>7){
     $adjustments['m']=1; //1 month
    } else if ($this->d>1){
     $adjustments['d']=7; //1 week
    } else {
     $adjustments['d']=1; //1 day
    }

    foreach($adjustments as $k=>$v){
     $this->$k=$v; //reset all the class properties
    }  
}

Any idea why? As far as the documentation seems to suggest, 'days' should be valid.

Answer 1

Actually, if you look more closely, DateInterval really DOESN'T have a class property named 'days'. These are "pseudo-properties", like the attributes on a SimpleXML object, and trying to extend these built-in classes from userland code isn't a straight-forward as you'd like it to be. Take a look at the Reflection class and its friends for help.