$_SESSION["id"] is always zero in the database - php

Question

I have been trying to solve this issue couldn't, tried reading from asked questions but couldn't get it. I started the session and required it in another page but when ever i want to use it to insert some text base on the current session id, it is always zero in the database. Please,explain to me, maybe i miss understood it. Thanks!
<?php
ob_start();
session_start();
if(isset($_SESSION['$user_id']) && !empty($_SESSION['$user_id']))
{
return true;
}
else
return false;
?>
include.php
<?php
include ("login._form.php");
require ("include.php");
require("require.php");
if ($_SERVER["REQUEST_METHOD"]== "POST")
{
$username = mysqli_real_escape_string($link, $_POST["user"]);
$password = mysqli_real_escape_string($link,$_POST["password"]);
if(empty($username) || empty($password))
{
die();
}
$row = mysqli_query($link,"SELECT * FROM `users` WHERE username ='$username'");
if($row === false)
{
echo "Query Error";
}
while($fetch = mysqli_fetch_array($row)){
if($username == $fetch["username"] && $password == $fetch["password"])
{
$_SESSION["id"] = $id;
header('Location:index.php');
}
else
die("user does'n exist");
}
mysqli_close($link);
}
login.php
?>
require("include.php");
include ("yd_sendpage_form.php");
require("require.php");
if ($_SERVER["REQUEST_METHOD"]== "POST")
{
$user_id = $_SESSION["id"];
$text = mysqli_real_escape_string($link,$_POST["text"]);
if(empty($text))
{
die("Field Can't Be Empty!");
}
$insert = mysqli_query($link,"INSERT INTO `text`(`id`, `user_id`, `text`) VALUES ('$user_id','$user_id','$text')");
}
?>
yd_sendpage.php

Answer 1

Change
if(isset($_SESSION['$user_id']) && !empty($_SESSION['$user_id']))
to
if(!empty($_SESSION['id']))
And don't forget about SQL injection, your code is vulnerable to it.
More information about SQL injection in your code

Answer 2

$_SESSION["id"] was not set, so i changed $_SESSION["id"] to $_SESSION["user"] = $username in the login.php and other pages to $_SESSION["user"] and that works. Thanks!