Class support unique param (id)

ClientUniq.php

abstract class ObjectWithUniqueID  {

    public $id;
    public static $registry = array();

    public function getObjectById( $id ) {
        if ( array_key_exists( $id, static::$registry ) ) {
            return static::$registry[$id];
        } else return null;
    }
}

class ClientUniq extends ObjectWithUniqueID {

    public $agreements = array();
    public static $registry = array();

    protected function __construct( $client_id, $client_name, $client_source ) {
        $this->id = $client_id;
        $this->client_name = $client_name;
        $this->client_source = $client_source;
        static::$registry[$client_id] = $this;
    }

    public static function getOrCreate( $client_id, $client_name, $client_source ) {
        $obj = static::getObjectById($client_id);
        if ( $obj === null ) {
            return new ClientUniq($client_id, $client_name, $client_source);
        } else return $obj;
    }
}

/* Usage

$a = ClientUniq::getOrCreate('1', 'test1', 'mysource');
array_push($a->agreements, 1);
$b = ClientUniq::getOrCreate('2', 'test2', 'mysource');
array_push($b->agreements, 2);
$c = ClientUniq::getOrCreate('2', 'test2', 'mysource');
array_push($c->agreements, 3);

var_dump(ClientUniq::$registry);

// a - object with agreement list [1]
// b and c - reference to the same object, and it has agreement list [2,3]

*/