从函数PHP调用函数

Question

Can anyone explain to me what is actually going on in the function?

function f($b=0)
{
    echo $b.',';
    /*otuput is 0,1,2,3,4,5,3,4,2,3,4,5,3,*/

    if ($b<3)
    {
        /*echo $b.',';
        when echo is here, otuput is 0,1,2,2,*/

        for ($a=0;$a<3;$a++)
        {
            /*echo $b.',';
            when echo is here, otuput is 0,1,2,3,4,2,3,1,2,3,4,2,*/

            f(++$b);
        }
    }
}

f();

Why is output not 0,1,2,0,1,2,0,1,2,? And how to do with the calling same function from function?

Answer 1

First of all it calls f(0) so it displays 0.

Then it calls f(1), f(2) & f(3) respectively.

f(1): It displays 1 and runs f(2), f(3) & f(4).

f(2): It displays 2 and runs f(3), f(4) & f(5).

f(3): It displays 3 only

f(4): It displays 4 only

f(5): It displays 5 only

...

So:

f(0):
    {
    0,
    f(1): 
        {
        1, 
        f(2): 
            {
            2, 
            f(3): 
                {
                3
                }, 
            f(4): 
                {
                4
                }, 
            f(5): 
                {
                5
                } 
            },
        f(3):
            {
            3
            },
        f(4):
            {
            4
            }
        },
    f(2):
        {
        2, 
        f(3): 
            {
            3
            }, 
        f(4): 
            {
            4
            }, 
        f(5): 
            {
            5
            } 
        },
    f(3):
        {
        3
        }
    }

So the output is: 0,1,2,3,4,5,3,4,2,3,4,5,3,