Question

I was wondering how php processes if statements.
If i were to have something such as:

if (isset($_GET['foo']) && $_GET['foo'] != $bar)

If foo isn’t set, would it then drop out of the if straight away (as it is an ‘and’ statement so it can’t succeed anyway) or would it also check the second part, rather pointlessly?

Answer 1

What you’re describing is known as “short-circuit evaluation”.

Most languages work this way, including PHP, so they will evaluate an expression until they are certain of the result, and then stop, so the remainder of the expression would not be evaluated.

As you say, this is the most efficient approach.

However, it can potentially throw a spanner in the works for inexperienced programmers, who nay try something like this:

if(doFirstProcess() && doSecondProcess() {
    print "both processes succeeded";
}

In this case, the programmer is expecting both functions to be called, but if the first one returns false, then the second one will not be executed, as the program already knows enough to be certain of the final result of the expression, so it short-circuits the remainder of the expression.

There are a few languages which don’t do short-circuit evaluation. VB6 was one example (back in the day). I don’t know about VB.Net, but since it’s evolved from VB6, I would suspect it would be similar. But aside from that, all other languages that I’ve worked with have used short-circuit evaluation, including PHP.

There is a section in the PHP manual about this here: http://www.php.net/manual/en/language.operators.logical.php

And you can read more on short circuit evalution here: http://en.wikipedia.org/wiki/Short-circuit_evaluation

Hope that helps.