What are some potential pitfalls when using global variables and variable variables in PHP code?

Potential pitfalls when using global variables and variable variables in PHP code include the risk of namespace collisions, making code harder to debug and maintain, and decreasing code readability. To mitigate these issues, it's recommended to limit the use of global variables and variable variables, and instead rely on passing variables as parameters to functions or using object-oriented programming techniques.

// Avoid using global variables and variable variables in PHP code
function addNumbers($num1, $num2) {
    return $num1 + $num2;
}

// Example usage
$number1 = 10;
$number2 = 20;
$result = addNumbers($number1, $number2);
echo $result;