What are the best practices for handling return values and global variables in recursive functions in PHP?

When dealing with return values and global variables in recursive functions in PHP, it is best practice to pass values as parameters to the recursive function and return them at each level of recursion. This helps maintain the integrity of the function and prevents unexpected behavior due to shared global state. Additionally, using return values allows for better control over the flow of the recursive function.

// Example of a recursive function that calculates the factorial of a number
function factorial($n, $result = 1) {
    if ($n == 0) {
        return $result;
    } else {
        return factorial($n - 1, $result * $n);
    }
}

// Usage
$number = 5;
$factorial = factorial($number);
echo "Factorial of $number is $factorial";