PHP operator precedence “Undefined order of evaluation”?

Question

http://www.php.net/manual/en/language.operators.precedence.php#example-115

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Answer 1

Operator precedence in PHP is a mess, and it's liable to change between versions. For that reason, it's always best to use parentheses to group your in-line equations so that there is no ambiguity in their execution.

The example I usually give when asked this question is to ask in turn what the answer to this equation would be:

$a = 2;
$b = 4;
$c = 6;
$val = $a++ + ++$b - 0 - $c - -++$a;

echo $val;

:)

Depending where I run it now, I get anything between 4 and 7, or a parser error.

This will load $a (1) into memory, then load it into memory again and increment it (1 + 1), then it will add the two together, giving you 3:

$a = 1;
$val = $a + ($a++);

This, however, is a parser error:

$a = 1;
$val = ($a + $a)++;

Anyway, long story short, your example 2) is the way that most versions will interpret it unless you add parenthesis around ($a++) as in the example above, which will make it run the same way in all PHP versions that support the incrementation operator. :)