PHP Fabonacci数列

<?php
    #Fibonacci数列问题
    #f(1) = 1
    #f(n) = f(n - 1) + f(n - 2)
    #求f(n)

    #递归方法
    function fibonacci($n) {
        if ($n == 0) {
            return 0;
        }

        if ($n == 1) {
            return 1;
        }

        return fibonacci($n - 1) + fibonacci($n - 2);
    }

    #非递归方法
    function fibonacci_norec($n) {
        $a = 0;
        $b = 1;

        for ($i = 2; $i <= $n; $i++) {
            $c = $a + $b;
            $a = $b;
            $b = $c;
        }

        return $c;
    }

    echo fibonacci(10);
    echo "<br>";
    echo fibonacci_norecr(10);
?>