# What is a cookie?

A **cookie** is a small file with a maximum size of 4KB. It is used to store user identification data. 

> Cookies are stored on the user's local machine.

Each time a browser page is requested, the computer sends the cookie alongside for authentication. Cookies are only visible to the user who creates them.

# Why are cookies used?

Since HTTP is a stateless protocol, cookies are used to track the state of the application. They are also used to track websites visited by the user to store user preferences.

# Creating cookies with PHP

We can create a cookie as follows:

```PHP
setcookie(name, value, expire, path, domain, secure, httponly);
```
> The `name` parameter is required. The rest are optional.

# Example

The code snippet below shows how we can create a cookie in PHP:

> The following cookie will expire in 30 days (86400s * 30).

<!DOCTYPE html>
<?php
$cookie_name = "user";
$cookie_value = "John Doe";
setcookie($cookie_name, $cookie_value, time() + (86400 * 30), "/"); // 86400 = 1 day
?>

We can also retrieve the set cookie using the `cookie_name` variable. It is done as follows:

<html>
<body>
<?php
  echo "Cookie '" . $cookie_name . "' is set!<br>";
  echo "Value is: " . $_COOKIE[$cookie_name];

?>
</body>
</html>

> Cookies are stored in the `_COOKIE` variable in PHP. Their value can be retrieved using the `cookie_name` variable, which acts as an identifier.

# Deleting a cookie

We can delete an existing cookie by changing its expiry date. We can set the expiry date to be before the current time. 

> The `setcookie` function needs to be used again for this purpose.

The code snippet below shows how to delete the same cookie that we created before:

<?php
// set the expiration date to one hour ago
setcookie("user", "", time() - 3600);
?>

# What is a session variable?

A **session variable** stores information that can be used across web pages. Session variables are stored on the browser. Therefore, a session lasts till the webpage is closed. 

# Creating a session

A session begins by using the `session_start` function. Each session variable is stored and retrieved from the global variable named `_SESSION`. Session variable are then created as follows: 

```PHP
$_SESSION[identifier] = value;
```
## Example

The example below shows how one can create session variables in PHP:

 <?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>

<?php
// Set session variables
$_SESSION["name"] = "Educative";
$_SESSION["ID"] = "123";
?>

</body>
</html> 

The code snippet below shows how we can retrieve session variables in PHP:

 <?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>

<?php
// Echo session variables that were set on previous page
echo "Company name is " . $_SESSION["name"] . ".<br>";
echo "Company ID is " . $_SESSION["ID"] . ".";
?>

</body>
</html> 

# Deleting a session

A session can be using the `session_unset` and `session_destroy` functions in PHP.

> The `session_unset` function removes all session variables.

> The `session_destroy` function destroys the session.

The example below shows how to destroy a session in PHP:

<?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>

<?php
// remove all session variables
session_unset();

// destroy the session
session_destroy();
?>

</body>
</html> 

What are cookies and sessions in PHP?

Cookies store user data for tracking, while session variables store information across web pages and last till the browser is closed.