Factorial Recursive

function factorial($n){
    return ($n == 1 || $n == 0) ? 1 : factorial($n - 1) * $n;
}

// Test

//2! = 2.1 = 2
echo factorial(2);
//3! = 3.2.1 = 6
echo factorial(3);
//4! = 4.3.2.1 = 24
echo factorial(4);
//5! = 5.4.3.2.1 = 120
echo factorial(5);