PHP: $_SESSION doesn't set - php

Question

Before setting as duplicate, I've spent 4 hours on researching about my problem, but I had no luck.
I am trying to make a signup/login system for my website. The main point that doesn't seem to work is that when I am signing up on my website, the session doesn't seem to start. The reason that I can see it is because, on my navbar, I have set it to change from signup to log out. Here is the piece of code for that:
<ul>
<li class="list1">Home</li>
<li class="list2">About</li>
<li class="list3">Portfolio</li>
<li class="list4">Blog</li>
<li class="list4">Contact</li>
<?php
if (isset($_SESSION['id'])){
echo "<li><a href='#'>SIGN OUT</a></li>";
}
else{
echo "<li><a onclick='signup(event)' href='#'>SIGN UP</a></li>";
}
?>
</ul>
To make that I have created three files. One is the mane page, one is the signup file itself, code below:
<?php
session_start();
include "../dbh.php";
$first = $_POST["first"];
$last = $_POST["last"];
$uid = $_POST["uid"];
$email = $_POST["email"];
$pwd = $_POST["pwd"];
$sql = "INSERT INTO users (first,last,uid,email,pwd) VALUES ('$first','$last','$uid','$email','$pwd')";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_assoc($result);
$_SESSION['id'] = $row['id'];
header("Location: ../index.php");
exit();
and the last one is the file which connects PHP to the database code below:
$conn = mysqli_connect("XXX","XXX","XXX","XXX");
if (!$conn){
die("Connection failed: ".mysqli_connect_error());
}
I believe that the session doesn't start because the main page reloads after the user hits signup on the form, but I have started the session on all of my files (except the database connection file where it's not needed). I used session start on all of my page and I placed it on the beginning of all pages with opening and closing PHP tags.
Any suggestions? I appreciate your answers and comments!
Sorry for the bad English but it's not my first language.

Answer 1

This:
$sql = "INSERT INTO users (first,last,uid,email,pwd) VALUES ('$first','$last','$uid','$email','$pwd')";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_assoc($result);
^^^^^^^^^^^^^^^^^^
$_SESSION['id'] = $row['id'];
Insert queries do NOT return a result set, and you can NOT fetch() from them. That means mysqli_fetch_assoc() is failing, and returning a boolean FALSE. You then use that boolean false as if it was an array, and are basically doing the equivalent of
$_SESSION['id'] = null;
Note this:
php > $foo = false;
php > $id = $foo['id'];
php > var_dump($id);
NULL
You want
$_SESSION['id'] = mysqli_insert_id($conn);
instead.

Answer 2

It is an error with you SQL query.
$sql = "INSERT INTO users (first,last,uid,email,pwd) VALUES ('$first','$last','$uid','$email','$pwd')";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_assoc($result);
The first line of the code is an INSERT command. The second line executes this command by sending it to the server. If query is properly processed then MySQL server doesn't return you anything, so $result will equal to true. It wil not contain any data from the database. So you can't fetch it, what you try to do in the third line. Need to make a separate query for data.