Unidentified Array Key in $_GET PHP

Question

I'm trying to create a CRUD system in PHP linked with XML that contains all the info that I want to create a CRUD system with. I'm stuck with editing the basic infos because in this code:

<?php
$movies = simplexml_load_file('sadfg.xml');

if(isset($_POST['submitSave'])) {

    foreach($movies->movie as $movie){
        if($movie['id']==$_POST['id']){
            $movie->movie_code = $_POST['movie_code'];
            $movie->title = $_POST['title'];
            $movie->directors = $_POST['directors'];
            $movie->genre = $_POST['genre'];
            $movie->date_of_release = $_POST['date_of_release'];
            break;
        }
    }
    file_put_contents('sadfg.xml', $movies->asXML());
    header('location:index.php');
}

foreach($movies->movie as $movie){
    if($movie['id']==$_GET['id']){
        $id = $movie['id'];
        $movie_code = $movie->movie_code;
        $title = $movie->title;
        $directors = $movie->directors;
        $genre = $movie->genre;
        $date_of_release = $movie->date_of_release;
        break;
        return;
    }
}
?>

if($movie['id']==$_GET['id']) -> this line says unidentified array key

This is my XML file:

<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="style.css"?>
<movies>
    <movie id="m1">
        <movie_code>MOV 1089</movie_code>
        <title>Guardians of the Galaxy</title>
        <directors>James Gunn</directors>
        <genre>Superhero fiction, Sci-Fi, Action</genre>
        <date_of_release>July 31, 2014</date_of_release>
    </movie>
    <movie id="m2">
        <movie_code>MOV 1145</movie_code>
        <title>Captain America: Civil War</title>
        <directors>Anthony Russo, Joe Russo</directors>
        <genre>Superhero fiction, Sci-Fi, Action</genre>
        <date_of_release>April 27, 2016</date_of_release>
    </movie>
    <movie id="m3">
        <movie_code>MOV 1220</movie_code>
        <title>Black Panther</title>
        <directors>Ryan Coogler</directors>
        <genre>Superhero fiction, Sci-Fi, Action</genre>
        <date_of_release>February 16, 2018</date_of_release>
    </movie>
    <movie id="m4">
        <movie_code>MOV 1265</movie_code>
        <title>The Avengers: Infinity War</title>
        <directors>Anthony Russo, Joe Russo</directors>
        <genre>Superhero fiction, Sci-Fi, Action</genre>
        <date_of_release>April 25, 2018</date_of_release>
    </movie>
    <movie id="m5">
        <movie_code>MOV 1331</movie_code>
        <title>Thor: Ragnarok</title>
        <directors>Taika Waititi</directors>
        <genre>Superhero fiction, Sci-Fi, Action</genre>
        <date_of_release>October 25, 2017</date_of_release>
    </movie>
</movies>

Stuck here for a while now, I wonder why this line says that. I can't seem to find the error causing it. Appreciate your thoughts or opinions guys thanks!