MySQL Join two Tables, give all entries of table 1

I have three tables.

Table 1: tbl_user

Table 2: tbl_location

Table 3: tbl_userlocation

Now I have a HTML-Page to edit user data

I have two querys. One for reading the userdata and one for the locations.

public function readOneUser($id)
{
    $stmt = $this->pdo->prepare("SELECT * FROM tbl_user WHERE id = :id");
    $stmt->execute([
        'id' => $id
    ]);
    $stmt->setFetchMode(PDO::FETCH_CLASS, "administration\\CMR\\UserModel");
    $res = $stmt->fetch(PDO::FETCH_CLASS);
    return $res;
}

That's fine. All is good. I have the normal userdata like username, mail, phone...

Now I want a complete list of all locations, BUT (and that's the problem) all locations where the user is assigned should be checked. Locations where the user is not assigned should not be checked but listed.

<?php
    foreach ($readLocations AS $location):
    ?>
    <input type="checkbox" name="location" <?= htmlspecialchars($location->id == $location->fk_location) ? "checked" : "" ?> value="<?= $location->id; ?>"><span><?= $location->location; ?></span><br>
    <?php
    endforeach;
?>

I have this query. I know it's wrong, but that's the "best" I have. Hope you can help me to make it correct.

public function readAllLocationsForEdit($id)
{
    $stmt = $this->pdo->prepare("
        SELECT tbl_location.*, tbl_userlocation.*
        FROM tbl_location
        LEFT JOIN tbl_userlocation ON tbl_userlocation.fk_location = tbl_location.id 
        WHERE fk_user = :id
        GROUP BY location");
    $stmt->execute([
        'id' => $id
    ]);
    $res = $stmt->fetchAll(PDO::FETCH_CLASS, "administration\\CMR\\LocationModel");
    return $res;
}

With this I get all locations where a user is assigned, but not the locations where the user is not assigned. I know, that the WHERE-clause is not correct, but all other I tested give me not the result I want.

The WHERE clause is applied after the JOIN.

For your needs, you need to filter the userlocation map either before or during the join. Either include the user filter in the join predicate, or join on a sub-query that filters the user (the former being the usual, and cleaner, approach).

     FROM tbl_location
LEFT JOIN tbl_userlocation
       ON tbl_userlocation.fk_location = tbl_location.id 
      AND tbl_userlocation.fk_user     = :id

Or...

     FROM tbl_location
LEFT JOIN (SELECT * FROM tbl_userlocation WHERE fk_user = :id) AS tbl_userlocation 
       ON tbl_userlocation.fk_location = tbl_location.id