PHP || and && logical optimization

Question

I'm a bit of an optimization freak (at least by my definition) and this question has been bugging me for quite a while.

I'm wondering if PHP does some optimization on && and ||: Take the following example:

$a = "apple";
$b = "orange";
if ($a == "orange" && $b == "orange") {
    //do stuff
}

When that code executes, it will check if $a is equal to "orange." In this case it isn't. However, there is an && operator. Since the first part ($a == "orange") already returned false, will PHP still check if $b is equal to "orange?"

I have the same question for ||:

$a = "orange";
$b = "orange";
if ($a == "orange" || $b == "orange") {
    //do stuff
}

When it checks if $a is equal to "orange," it returns true. Since that would make the || operator return true, will PHP even check the second part of the || (since we already know it will be true)?

Hopefully I am making sense here, and hopefully somebody has an answer for me. Thank you!

Answer 1

PHP uses short circuit evaluation with binary conditionals (such as &&, || or their constant equivalents), so if the result of evaluating the LHS means the RHS isn't necessary, it won't.

For example...

method_exists($obj, 'func') AND $obj->func();

...is an exploitation of this fact. The RHS will only be evaluated if the LHS returns a truthy value in this example. The logic makes sense here, as you only want to call a method if it exists (so long as you're not using __call(), but that's another story).

You can also use OR in a similar fashion.

defined('BASE_PATH') OR die('Restricted access to this file.');

This pattern is used often as the first line in PHP files which are meant to be included and not accessed directly. If the BASE_PATH constant does not exist, the LHS is falsy so it executes the RHS, which die()s the script.