How can the use of global variables impact the functionality of PHP scripts, and what best practices should be followed to avoid errors related to undefined variables?

Using global variables in PHP scripts can impact functionality by making code harder to debug and maintain. It can lead to unexpected behavior if variables are overwritten unintentionally. To avoid errors related to undefined variables, it's best practice to limit the use of global variables and instead pass variables as arguments to functions where they are needed.

// Avoid using global variables and pass variables as arguments to functions

function calculateTotal($price, $quantity) {
    $total = $price * $quantity;
    return $total;
}

$price = 10;
$quantity = 5;

$total = calculateTotal($price, $quantity);
echo "Total: $total";