How can the use of global variables in PHP scripts impact the functionality and security of the code, and what are the best practices for handling them?

Using global variables in PHP scripts can impact functionality by making code harder to read and maintain. It can also pose security risks, as global variables can be easily manipulated by malicious users. To handle global variables more effectively, it's best to avoid using them whenever possible and instead use function parameters or return values to pass data between functions.

// Avoid using global variables and pass data through function parameters
function calculateTotal($price, $quantity) {
    return $price * $quantity;
}

$total = calculateTotal(10, 5);
echo $total;