Recursion vs. iteration in PHP

Php is a special case. You will use less memory using the iterative solution. Moreover, function calls in PHP are costly, so it's better to avoid function calls when you can.

PHP will seg fault (on my system) trying to find the factorial of 100,000, but the iterative solution has no problems. Both of them execute practically instantaneously, though.

Of course, factorial of a much smaller number is INF, but this could also be applied to a much slower growing function.

If we're not talking about PHP or another scripting langauge, then there is no standard. It's good to know how to do it both ways. I would go with whichever leads to the cleanest code.

What I don't know is which method is better to used and why?

Rule of thumb: If you can write it iteratively, use that. This is because function calls and recursion come with some penalty in PHP. Also, PHP doesn't natively come with recursion protection and you'd risk running out of memory on big numbers.

What's the industry standard?

As far as I know, there's no industry standard for calculating a factorial. In general, industry standards don't go into such detail; if they do, then great.

How can I select one of the methods between above two?

Independent of the true nature of your functions, you can reach a conclusion as to which is better by yourself as well, simply by running the functions over the input domain and benchmarking both.

What's the condition to determine which one is better?

Memory and time are important factors. You should try to achieve low memory consumption AND short execution time. This is not always possible, in which case you need to compromise either one.

That said, I would opt for an iterative solution.

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Btw, if PHP were to ever implement tail recursion optimization, you could write the factorial function like so:

function fact($n, $r = 1)
{
    if ($n < 2) {
        return $r;
    } else {
        return fact($n - 1, $r * $n);
    }
}