How to Fix PHP Login Errors (Session Not Persisting / Login Failure)

1. Problem Description
When implementing a login feature in PHP, you may run into issues like these:

Getting redirected back to the login page even after logging in
Login fails despite entering the correct username and password
Login succeeds, but refreshing the page logs you out
Session is not being maintained

These problems occur not only for beginner developers, but also in real production environments.

2. Root Causes
Login errors in PHP are most commonly caused by the following:
(1) Missing session_start()
To use sessions, you must call session_start() at the very top of every page that uses session data.
(2) Database Connection Error
If the database connection fails, the login validation process itself cannot proceed.
(3) Incorrect Password Comparison Method
Comparing an encrypted (hashed) password using a simple equality check will always fail.
(4) Cookie Configuration Issues
If cookies are blocked in the browser, sessions cannot be maintained.

3. Solutions
Here is how to address each cause:
✔ Add session_start()
This must be placed at the very top of all login-related pages — before any output.
✔ Verify the Database Connection
Confirm that your mysqli or PDO connection is established successfully.
✔ Use password_hash() and password_verify()
Passwords must be hashed when stored and verified using the proper function — never compared as plain text.
✔ Check Cookie Settings
Make sure that cookies are allowed in the user's browser.

4. Code Example
Below is a basic PHP login handler:
php<?php
session_start();

$conn = new mysqli("localhost", "user", "password", "dbname");

$id = $_POST['id'];
$pw = $_POST['pw'];

$sql = "SELECT * FROM users WHERE id = ?";
$stmt = $conn->prepare($sql);
$stmt->bind_param("s", $id);
$stmt->execute();

$result = $stmt->get_result();
$user = $result->fetch_assoc();

if ($user && password_verify($pw, $user['password'])) {
$_SESSION['user_id'] = $user['id'];
echo "Login successful";
} else {
echo "Login failed";
}
?>

5. Summary
Most PHP login errors come down to basic configuration mistakes. Make sure to:

Always call session_start()
Hash passwords and compare them properly
Verify the database connection is working
Check that cookies are enabled

Getting these four things right will resolve the vast majority of login issues.

💡 Pro Tip
When building a login system, security should be a priority from the start. Make sure to implement SQL injection prevention (e.g., prepared statements), enforce HTTPS, and apply other security best practices alongside your core login logic.