In mathematics, Fibonacci numbers are special integer sequences where a number is composed from summation of the past two numbers, as shown in the following the expression:

If we implement this using PHP 7, it will look like this:

function fibonacci(int $n): int { 
    if ($n == 0) { 
    return 1; 
    } else if ($n == 1) { 
    return 1; 
    } else { 
    return fibonacci($n - 1) + fibonacci($n - 2); 
    } 
}

If we consider the preceding implementation, we can see it is a bit different from the previous examples. Now, we are calling two functions from one function call. We will discuss different types of recursions shortly.