Undefined variable id

suhu-suhu yang saya hormati, kan aku nemu eror di sini ya

katanya kan undifined variable id, nah perasaan kan saya udah define si variable nya disini nih, tapi kok masih eror ya?

 <?php
require_once "core/init.php";

$error = '';
$id = $_GET['id'];

if(isset($_GET['id'])){
	$article = tampilkan_per_id($id);
	while($row = mysqli_fetch_assoc($article)){
		$judul_awal = $row['judul'];
		$konten_awal = $row['isi'];
		$tag_awal = $row['tag'];
	}
}

if(isset($_POST['submit'])){
	$judul  = $_POST['judul'];
	$konten = $_POST['konten'];
	$tag    = $_POST['tag'];

	if(!empty(trim($judul)) && !empty(trim($konten))){

		if(edit_data($judul, $konten, $tag, $id)){
			 header('Location: index.php');
		}else{
			$error = 'masalah pas ngupdate data';
		}

	}else{
		$error = 'judul dan kontennya harus diisi';
	}
}

?>

<form action="" method="post">
	<label for="judul">Judul</label><br>
	<input type="text" name="judul" value="<?=$judul_awal; ?>"><br><br>

	<label for="konten">Isi</label><br>
	<textarea name="konten" rows="20" cols="100"><?=$konten_awal; ?></textarea><br><br>

	<label for="tag">Tag</label><br>
	<input type="text" name="tag" value="<?=$tag_awal; ?>"><br><br>

	<div id="error"><?=$error ?></div>

	<input type="submit" name="submit" value="submit">
</form>

nah ini function2-nya

 function tampilkan(){
	global $link;

	$query = "SELECT * FROM blogpost";
	$result = mysqli_query($link, $query) or die ('gagal nampilin data');

	return $result;
}

function tampilkan_per_id($id){
	global $link;

	$query = "SELECT * FROM blogpost WHERE id=$id"; //perintah ke database
	$result = mysqli_query($link, $query);

	return $result;
}

function tambah_data($judul, $konten, $tag){
	$query = "INSERT INTO blogpost (judul, isi, tag) VALUES ('$judul', '$konten', '$tag')";
	return run($query);
}

function edit_data($judul, $konten, $tag){
	$query = "UPDATE blogpost SET judul='$judul', isi='$konten', tag='$tag'
	WHERE id=$id";

	return run($query);
}

function run($query){
	global $link;

	if(mysqli_query($link, $query)) return true;
	else return false;
}

Mohon pencerahannya ya suhu :)