Why does this PHP snippet behave this way?

I'm just doing some recursion exercises in PHP and I'm a bit baffled by the output 开发者_如何学Cof the following:

function calc($numTimes, $i, $total) {
    if (!$i && !$total) {$i = 1; $total = 1;}
    if ($i <= $numTimes) {
        $total = $total*2;
        $i++;
        calc($numTimes, $i, $total);
    }
    echo $total.'+'.$i.'<br />';
}
calc(5);

Before running it I would've assumed the output to be 32+6. However, this is what I get:

32+6
32+6
16+5
8+4
4+3
2+2

I don't get it. The output is not only 5 lines longer than I would've expected it to be, but instead of incrementing the total, it's removing from it? Also, if I add a break; after the echo, it only returns 32+6, which somehow seems relevant. However, when I change the code so that it uses return $total; instead of echo:

function calc($numTimes, $i, $total) {
    if (!$i && !$total) {$i = 1; $total = 1;}
    if ($i <= $numTimes) {
        $total = $total*2;
        $i++;
        calc($numTimes, $i, $total);
    }
    return $total.'+'.$i.'<br />';
}
$r = calc(5);
echo $r;

This is what gets printed out:

2+2

I'm a bit confused and hoping someone can help me understand what's going on here.

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you are not doing anything with the recursive call. the line:

calc($numTimes, $i, $total);

might calculcate a value, but does nothing with it. notice that the returned value is never saved. you would have to fetch it:

$res = calc($numTimes, $i, $total);

and then keep going with $res

i think what you meant is:

function calc($numTimes, $i = 0, $total = 0) {
    if (!$i && !$total) {$i = 1; $total = 1;}
    if ($i <= $numTimes) {
        $total = $total*2;
        $i++;
        return calc($numTimes, $i, $total);
    }
    return $total.'+'.$i.'<br />';
}
echo calc(5);

---

In your first example, calc() is called inside itself conditionally, hence it's looping and outputting numerous results (5 calls to echo).

In your second example, you have set a variable to be the result of the return value of calc(). It is still looping, but the result is overridden each time. So you have one result showing (echo is called once).

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You just have a typo, using $sum instead of $total in the first if clause.