Mastering PHP: From Basics to Building Dynamic Web Applications

Chapter 6 : Sessions, Cookies & File Handling

🔹 1. Introduction

As users interact with your application, you need ways to:

• Remember who they are
• Store temporary or persistent data
• Read, write, or upload files

PHP provides built-in support for sessions, cookies, and file management — critical for login systems, personalization, and content-driven apps.

🔹 2. PHP Sessions – Storing Data Across Pages

Sessions store data on the server and assign a unique ID to each user. It's ideal for storing:

• Login status
• User preferences
• Temporary values between pages

✅ Starting a Session:

session_start(); // Must be at the top of the file

$_SESSION["username"] = "john";

echo $_SESSION["username"]; // Output: john

✅ Unsetting and Destroying:

unset($_SESSION["username"]); // remove one

session_destroy(); // clear all session data

⚠️ Always call session_start() before any HTML output.

🔹 3. PHP Cookies – Storing Data on the Client Side

Cookies are stored in the user's browser and are useful for:

• Remembering login (if permitted)
• Saving theme preferences
• Tracking visits

✅ Setting a Cookie:

setcookie("user", "john", time() + (86400 * 7)); // 7 days

✅ Reading a Cookie:

echo $_COOKIE["user"];

✅ Deleting a Cookie:

setcookie("user", "", time() - 3600); // Expired

🔹 4. File Uploads in PHP

To accept file uploads, your form must use enctype="multipart/form-data".

✅ Upload Form:

<form method="post" enctype="multipart/form-data">

  <input type="file" name="upload">

  <input type="submit" name="submit">

</form>

✅ Upload Handling:

if (isset($_FILES["upload"])) {

  $file = $_FILES["upload"];

  $name = $file["name"];

  $tmp = $file["tmp_name"];

  move_uploaded_file($tmp, "uploads/" . $name);

  echo "Uploaded!";

}

✅ Use is_uploaded_file() and validate the file size/type for security.

🔹 5. Reading and Writing Files in PHP

✅ Writing to a File:

$file = fopen("data.txt", "w");

fwrite($file, "Hello World!");

fclose($file);

✅ Reading a File:

$file = fopen("data.txt", "r");

$content = fread($file, filesize("data.txt"));

fclose($file);

echo $content;

🔹 6. File Existence & Deletion

✅ Check If File Exists:

if (file_exists("data.txt")) {

  echo "File exists.";

}

✅ Delete File:

unlink("data.txt");

🔹 7. Real-World Example: Visit Counter with File

$file = "counter.txt";

if (file_exists($file)) {

  $count = (int)file_get_contents($file);

} else {

  $count = 0;

}

$count++;

file_put_contents($file, $count);

echo "You are visitor #$count";

✅ file_get_contents() and file_put_contents() are simpler alternatives to fopen().

🔹 8. Session vs Cookie: When to Use What?

🔹 9. Summary Table