What are best practices for initializing variables before using them in PHP scripts, especially when dealing with input from external sources like files?

When dealing with input from external sources like files in PHP scripts, it is essential to initialize variables before using them to avoid potential errors or security vulnerabilities. One best practice is to always check if the variable is set before using it to prevent undefined variable errors. Additionally, you can assign default values to variables if they are not set or sanitize the input to ensure it is safe to use in your script.

// Example of initializing variables before using them with input from external sources

// Check if the variable is set before using it
if(isset($_POST['username'])){
    $username = $_POST['username'];
} else {
    $username = '';
}

// Assign default values to variables if they are not set
$email = isset($_POST['email']) ? $_POST['email'] : '';

// Sanitize the input to ensure it is safe to use
$cleanedInput = filter_input(INPUT_POST, 'input', FILTER_SANITIZE_STRING);