PHP运算符优先级为“未定义的评估顺序"? [英] PHP operator precedence "Undefined order of evaluation"?

问题描述

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

<?php
$a = 1;
echo $a + $a++; // may print either 2 or 3
?>

php手册中的示例不能很好地解释.为什么不将$a++评估为2，然后又将其添加到1，以使其始终成为echo 1 + 2 // equals 3?我不明白它可能打印2或3"的方式.我认为增量++的优先级比加法+的优先级高?

The example from the php manual doesn't explain very well. Why isn't $a++ evaluated to 2, and then added to 1, so that it always becomes echo 1 + 2 // equals 3? I don't understand how it "may print either 2 or 3". I thought incremental ++ has "higher precedence" than addition +?

换句话说，我不明白为什么不是...

In other words, I don't understand why isn't it...

$a = 1;

1) echo $a + $a++;
2) echo 1 + ($a = 1 + 1);
3) echo 1 + (2);
4) echo 3;

推荐答案

PHP中的运算符优先级是一团糟，并且很容易在版本之间进行更改.因此，最好使用括号对内联方程进行分组，以免执行时含糊不清.

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.

当我问这个问题时，我通常举的例子是依次问这个方程的答案是什么:

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;

:)

根据我现在在哪里运行，我会得到4到7之间的任何值，或者解析器错误.

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

这会将$ a(1)加载到内存中，然后再次将其加载到内存中 并递增(1 + 1)，然后将两者加在一起，得到3:

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++);

但是，这是解析器错误:

This, however, is a parser error:

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

总之，长话短说，您的示例2)是大多数版本将解释它的方式，除非像上面的示例中在($a++)周围加上括号，这将使其在所有PHP版本中以相同的方式运行支持增量运算符. :)

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. :)

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