变量前面的感叹号 - 需要澄清 [英] Exclamation mark in front of variable - clarification needed

问题描述

我已经使用PHP了很长一段时间了，但这对我来说总是一个谜，在变量前正确使用感叹号（负号）。

I've been working with PHP for quite a while now, but this was always a mystery to me, the correct use of the exclamation mark (negative sign) in front of variables.

！$ var 表示什么？ var false ，为空，未设置等等。

What does !$var indicate? Is var false, empty, not set etc.?

以下是我需要学习的一些示例.. 。

Here are some examples that I need to learn...

示例1：

$string = 'hello';
$hello = (!empty($string)) ? $string : '';

if (!$hello)
{
    die('Variable hello is empty');
}

这个例子有效吗？如果 $ string 为空，if语句是否真的有效？

Is this example valid? Would the if statement really work if $string was empty?

示例2：

$int = 5;
$count = (!empty($int)) ? $int : 0;

// Note the positive check here
if ($count)
{
   die('Variable count was not empty');
}

此示例是否有效？

我从不使用上述任何一个例子，我将这些 if（$ var）限制为只有布尔值的变量。我只需要知道这些示例是否有效，这样我就可以扩大 if（$ var）语句的使用范围。他们看起来很干净。

I never use any of the above examples, I limit these if ($var) to variables that have boolean values only. I just need to know if these examples are valid so I can broaden the use of the if ($var) statements. They look really clean.

谢谢。

推荐答案

if（！$ a）与相同if（$ a == false）。此外，应该考虑使用 == 运算符时发生类型转换。

有关详细信息，请查看与...的松散比较==此处部分。从那里开始，字符串0和等于FALSE（0== false 为TRUE且= = false 也是TRUE。

if(! $a) is the same as if($a == false). Also, one should take into account that type conversion takes place when using == operator.
For more details, have a look into "Loose comparisons with ==" section here. From there it follows, that for strings "0" and "" are equal to FALSE ( "0"==false is TRUE and ""==false is TRUE, too).

关于发布的示例：

示例1

它会起作用，但你应该注意，0和都是'空'字符串。

Regarding posted examples:
Example 1
It will work, but you should note, that both "0" and "" are 'empty' strings.

示例2

它将起作用

Example 2
It will work

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