$letter = new acceptance_letter( "student", system::get_value("schoolyear"));

$student->add_acceptance_letter( $letter );

$student->get_acceptance_letter( "student", system::get_value("schoolyear") );




	public class Acceptance_Letter
	{
		private $type;

		private $year;

		public __construct( $type, $year )
                {
                   $this->type = $type;
                   $this->year = $year;
                }

                public get_type(){ return type; }
                public get_year(){ return year; }

	}

....

in student

        public get_acceptance_letter( $type, $year )
        {
             foreach( $this->acceptance_letters as $letter )
             {
                 $the_type = $letter->get_type();
                 $the_year = $letter->get_year();

                 if( type == the_type && year == the_year )
                 {
                     // found it !
                     return $letter;
                 }
             }
        }