public function add_ref($item, $id) {
        $this->validate_item($item);
        $this->refs[$item][$id] = true;
    }
public function add_ref($item, $id) {
        $this->validate_item($item);
        $this->refs[$item][$id] = true;
    }
public function add_ref ( $ item , $ id ) { $ this -> validate_item ( $ item ) ; $ this -> refs [ $ item ] [ $ id ] = true ; }
Adds a reference

@param string $item the name of referenced item (like user, file, scale, outcome or grade_item)
@param int $id the value of the reference
Adds a reference