how to concatenate objects of a class in php?

I have the Message class:

class Mensaje {

    public $contenido  = "";
    public $destino    = "";
    public $estado     = -1;
    public $message_id = -1;

    function __construct( $destino, $contenido ) {

        $this->contenido = $contenido;
        $this->destino   = $destino;
    }

    function queue( 
        $url, 
        $api_user, 
        $api_pass, 
        $api_version 
    ) {
        $info = array(
            'api_version' => $api_version
            'cmd'         => 'api_queue_sms',
            'content'     => $this->contenido,
            'destination' => $this->destino,
            'password'    => $api_pass,
            'username'    => $api_user,
        );
        $args = urlencode( json_encode( $info ));
        echo $args;

        $res = file_get_contents( $url + '?' + $args ).read();
        $obj = json_decode( $res );

        if ( $obj[ 'success' ] ) {
            $mensajes[ $i ].$estado = 0;
            $this->message_id = $obj[ 'message_id' ];
            echo 'Mensaje insertado exitosamente. Ticket: ' + str( $obj[ 'message_id' ] );

            return 1;

        } else {
            $mensajes[ $i ].$estado = -1;
            echo 'Error al insertar mensaje. Codigo de error: ' + $obj[ 'error_code' ];

            return -1;       
        }
    }


} //class

I need to concatenate the objects created from this class. Code where I try to concatenate with append :

/*** Parte donde quiero concatenar ***/
for ( $c=0; $c < $numero; $c++ ) {

    $mensajes->append( new Mensaje( $datos[0], $datos[1] ));
    $mensajes[ $i ]->queue( $url, $api_user, $api_pass, $api_version );
    $i = $i + 1; 
}

Error I get:

> Call to a member function Mensaje() on null in C:\AppServ\www\test_msj_php.php on line 83