array_push（）期望参数1为数组，给定为null

I have the following php code:

<?php

class Session {

    public function __construct() {
        if (!session_id()) {
            session_start();
        }
        if (!is_array($_SESSION['messages'])) {
            $_SESSION['messages'] = [];
        }
        echo is_array($_SESSION['messages']);
    }

    public function AddMessage($msg="") {
        array_push($_SESSION['messages'], $msg);
    }

    public function GetMessages() {
        $messages = $_SESSION['messages'];
        unset($_SESSION['messages']);
        return $messages;
    }

}

$session = new Session();

?>

And the output of $session->AddMessage('message') is:

1 
Warning: array_push() expects parameter 1 to be array, null given in ...\Session.php on line 16

so is_array clearly says that $_SESSION['messages'] IS an array. What am I missing?

session_id is a function, so it should be:

    if (!session_id()) { //Not just session_id
        session_start();
    }

If you would have turned on error reporting it might have thrown:

Notice: Use of undefined constant session_id - assumed 'session_id'

Note: OP has edited the original question