Top Mistakes of Beginner PHP Developers: Advanced Level

Top Mistakes of Beginner PHP Developers: Advanced Level

When you move from simple scripts to complex web applications in PHP, the number of pitfalls increases dramatically. Many developers, having mastered the basic syntax, make systemic errors that lead to memory leaks, vulnerabilities, and performance degradation. In this article, we will look at 4 advanced mistakes often made by intermediate-level developers and show how to fix them.

1. Incorrect Session Management and Memory Leaks

One of the most insidious mistakes is incorrect session handling. Many developers store large amounts of data in sessions (e.g., results of database queries or serialized objects). This not only slows down performance but can also lead to memory leaks on the server.

Problem: The session is locked for the duration of the script execution. If you store 10 MB of data in it, each request will read and write this volume, and with parallel requests, they will queue up.

Example of bad code:

<?phpsession_start();// Loading an array of 10000 records into the session$_SESSION['big_data'] = $db->query("SELECT * FROM logs")->fetchAll();?>

Solution: Use sessions only for identification (user_id, token). Store all other data in cache (Redis, Memcached) or in the database.

<?phpsession_start();$_SESSION['user_id'] = $user->id;// Get data on demand$cache = new RedisCache();$data = $cache->get('user_data_' . $user->id);if (!$data) {    $data = $db->query("SELECT * FROM logs WHERE user_id = ?", [$user->id])->fetchAll();    $cache->set('user_data_' . $user->id, $data, 3600);}?>

2. Incorrect Caching and Ignoring Invalidation

The second most common mistake is caching "forever". Developers set the cache time-to-live (TTL) to 24 hours or more, and then wonder why users see outdated data. Even worse is caching without accounting for changes in the database.

Problem: If you cache a product list for 1 hour, and an administrator changes the price, users will see the old price for another 60 minutes.

Example of bad code:

<?php$cacheKey = 'product_list';$products = $cache->get($cacheKey);if (!$products) {    $products = $db->query("SELECT * FROM products")->fetchAll();    $cache->set($cacheKey, $products, 86400); // 24 hours}?>

Solution: Use cache tagging or forced invalidation when data changes.

<?php// When updating a productfunction updateProduct($id, $data) {    $db->update("products", $data, ["id" => $id]);    // Invalidate cache    $cache->delete('product_list');    $cache->delete('product_' . $id);}

// When reading$products = $cache->remember('product_list', 3600, function() use ($db) {    return $db->query("SELECT * FROM products WHERE active = 1")->fetchAll();});?>

3. SQL Injections via "Safe" Functions

Many developers believe that if they use mysqli_real_escape_string() or addslashes(), they are protected from SQL injections. This is a dangerous misconception. These functions do not protect against all types of attacks, especially when working with LIKE, ORDER BY, and LIMIT.

Problem: The mysqli_real_escape_string() function does not escape characters for the LIKE operator (e.g., % and _). If you insert user input into LIKE, an attacker can access data they should not see.

Example of vulnerability:

<?php$search = $_GET['q'];$safe = mysqli_real_escape_string($conn, $search);$query = "SEL