Wie lade ich Dateien in PHP korrekt hoch und gehe mit möglichen Fehlern um?

$folder = "upload/";
if (is_uploaded_file($HTTP_POST_FILES['filename']['tmp_name'])) {   
    if (move_uploaded_file($HTTP_POST_FILES['filename']['tmp_name'], $folder . $HTTP_POST_FILES['filename']['name'])) {
         echo "File uploaded";
    } else {
         echo "File not moved to destination folder. Check permissions";
    };
} else {s
     echo "File is not uploaded";
}; 

Notice: Undefined variable: HTTP_POST_FILES in C:\wamp\www\sdg\import\ips.php on line 3

$folder = "upload/";
if (is_uploaded_file($_FILES['filename']['tmp_name'])) {   
    if (move_uploaded_file($_FILES['filename']['tmp_name'], $folder . $_FILES['filename']['name'])) {
         echo "File uploaded";
    } else {
         echo "File not moved to destination folder. Check permissions";
    };
} else {s
     echo "File is not uploaded";
}; 

$target_dir = "upload/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$imageFileType = strtolower(pathinfo($target_file, PATHINFO_EXTENSION));
$allowedTypes = ['jpg', 'png'];

if (isset($_POST["submit"])) {
    // Check file type
    if (!in_array($imageFileType, $allowedTypes)) {
        $msg = "Type is not allowed";
    } 
    // Check if file exists
    elseif (file_exists($target_file)) {
        $msg = "Sorry, file already exists.";
    } 
    // Check file size
    elseif ($_FILES["fileToUpload"]["size"] > 5000000) {
        $msg = "Sorry, your file is too large.";
    } 
    // Move file
    elseif (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
        $msg = "The file " . basename($_FILES["fileToUpload"]["name"]) . " has been uploaded.";
    }
}
?>

<form action="upload.php" method="post">