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To: Hodicska Gergely <felho@avalon.aut.bme.hu>
Cc: internals@lists.php.net
In-Reply-To: <4183E133.1050103@avalon.aut.bme.hu>
References: <4183E133.1050103@avalon.aut.bme.hu>
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Organization: InterJinn
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Date: Sat, 30 Oct 2004 15:10:41 -0400
Subject: Re: [PHP-DEV] operator strange behaviour
From: robert@interjinn.com (Robert Cummings)

On Sat, 2004-10-30 at 14:45, Hodicska Gergely wrote:
> Hi!
> 
> I tried to get answer on the general list, but I didn't get.
> I have a little example, which generate strange output.
> 
> $a = 0;
> $b = 1;
> if ($a = 1 && $b = 0) {
>      echo 'true ';
>      var_dump($a);
>      var_dump($b);
> } else {
>      echo 'false ';
>      var_dump($a);
>      var_dump($b);
> }
> Runing this we get: "flase bool(false) int(0)"
> 
> After the precedence table the first step should be evaluating the &&, 
> and we get something like this: $a = false = 0, which should generate an 
> error, while there isn't an "lvalue" on the left side of the =.
> But doesn't this happen. It seems, that the evaluation of the first = 
> came before evaluating the &&.
> 
> Can someone exactly explain how PHP process this condition?

You are mixing right and left associativity operators without
parenthesis. The PHP doc at:

    http://www.php.net/manual/en/language.operators.php

Clearly states that you can do !$a = foo() where the result of foo()
will be assigned to $a. Thus in your example the order of evaluation is:

    evaluate $b = 0
    evaluate the result of 1 && $b
    assign the previous evaluation result to $a

Thus:

    $b = 0
    1 && 0 = false
    $a = false

HTH,
Rob.
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