PHP if execution

Question

In PHP if you have the following code does $b get evaluated since $a will cause the if statement to return false?

$a = false;
$b = true;

if ($a && $b) {
  // more code here
}

also if $b does get evaluated are there ever circumstances where a portion of an if statement may not be evaluated as the processor already knows that the value to be false?

php evaluates quite lazy. since $a is false, the expression will be false regardless of $b, so there is no reason to evaluate $b. (extensively covered in the manual and tutorials, AFAICR). also, you could've checked yourself by using functions that echo something and return a boolean. — Franz Gleichmann
– Franz Gleichmann, Commented Oct 18, 2016 at 9:26
Also with opcache there are cases when the entire if isn't executed. — PeeHaa
– PeeHaa, Commented Oct 18, 2016 at 9:27
In theory, you shouldn't assume an order of operations since future versions may change it and evaluate $b and then $a. In practice everyone already assumes that $b is not evaluated so it's doubtful that this change will ever happen, so as it stands now, $a is evaluated and if it's false then the whole if fails (without evaluating $b) — apokryfos
– apokryfos, Commented Oct 18, 2016 at 9:50

Alok Patel · Accepted Answer · 2016-10-18 09:30:34Z

4

Evaluation of && is stopped as soon as it hits the false condition.

These (&&) are short-circuit operators, so they don't go to check second condition if the first one true (in case of OR) or false(in case of AND).

Reference: http://php.net/manual/en/language.operators.logical.php

From documentation:

<?php

// --------------------
// foo() will never get called as those operators are short-circuit

$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());

answered Oct 18, 2016 at 9:30

Alok Patel

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Al Foиce ѫ · Accepted Answer · 2016-10-18 15:17:44Z

1

Evaluation of logical expressions is stopped as soon as the result is known.

If $a is false, $b will not get evaluated, as it won't change the ($a && $b) result.

A consequence of that is that if the evaluation of $b requires more ressources than the evaluation of $a, starts your test condition with $a.

But be aware that:

PHP does not (in the general case) specify in which order an expression is evaluated and code that assumes a specific order of evaluation should be avoided, because the behavior can change between versions of PHP or depending on the surrounding code. (Source php docs)

So you should not assume $b is never evaluated if $a is false, as it may change in the future (says php docs).

edited Oct 18, 2016 at 15:17

answered Oct 18, 2016 at 9:26

Al Foиce ѫ

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1 Comment

Morgan Richards Over a year ago

Thanks for the clariffication useful to know

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