Session Won't Start?

Hi all,

I'm trying to code a login script but the new user session won't start and just returns to login page -

<?php
session_start();

require_once "db.php";

if(isset($_SESSION['user_id'])!="") {
    header("Location: dashboard.php");
}

if (isset($_POST['login'])) {
    $email = mysqli_real_escape_string($conn, $_POST['email']);
    $password = mysqli_real_escape_string($conn, $_POST['password']);

    if(!filter_var($email,FILTER_VALIDATE_EMAIL)) {
        $email_error =...

Code (markup):

Session Won't Start?

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