//file cp.php
class cp {

	...	
	function get_id_cp() {
		return $this->id_cp;
	}
...
}

//file zone.php

class zone {
..
function get_id_zone() {
		return $this->id_zone;
	}

...
}

//file zone_cp.php
class zone_cp {
	
	var $cp;
	var $zone;
	
	function zone_cp($zone,$cp) {
		global $zone,$cp;
		$this->setZone($zone);
		$this->setCp($cp);
		print_r($this);
		echo dirname(__FILE__);
		$this->print_();
	}

function print_() {
		echo $this->zone->get_id_zone(); //error here : Call to a member function on a non-object
	}
	...
}