What are common methods for sending variables in PHP, and what are the best practices for each?

When sending variables in PHP, common methods include using GET or POST requests, session variables, cookies, and using hidden form fields. Best practices for each method include properly sanitizing and validating input data to prevent security vulnerabilities, using encryption for sensitive data, and avoiding sending sensitive information via GET requests due to visibility in the URL. 

// Using POST request to send variables securely
<form method="post" action="process.php">
    <input type="text" name="username">
    <input type="password" name="password">
    <button type="submit">Submit</button>
</form>

// Using session variables to store and retrieve data
<?php
session_start();
$_SESSION['user_id'] = 123;
$user_id = $_SESSION['user_id'];
?>

// Using cookies to send variables
<?php
setcookie('user_id', 123, time() + 3600, '/');
$user_id = $_COOKIE['user_id'];
?>

// Using hidden form fields to send variables
<form method="post" action="process.php">
    <input type="hidden" name="user_id" value="123">
    <button type="submit">Submit</button>
</form>

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