null coalescing operator in PHP, PHP null coalescing operator_PHP tutorial

null coalescing operator in PHP, PHP null coalescing operator

<code class="meta">project: blog
target: null-coalesce-operator-in-php.md
date: 2015-12-30
status: publish
tags:
    - Null Coalesce
    - PHP
categories:
    - PHP
</code>

The null coalescing operator is a good thing. With it, we can easily get a parameter and provide a default value when it is empty. For example, you can use || in js:

<code class="js">function setSomething(a){
    a = a || &#39;some-default-value&#39;;
    // ...
}
</code>

In PHP, unfortunately, PHP's || always returns true or false, so it cannot be done this way.
PHP7 has only officially added the ?? operator:

<code class="php">// 获取user参数的值(如果为空，则用&#39;nobody&#39;)
$username = $_GET[&#39;user&#39;] ?? &#39;nobody&#39;;
// 等价于:
$username = isset($_GET[&#39;user&#39;]) ? $_GET[&#39;user&#39;] : &#39;nobody&#39;;
</code>

It is estimated that PHP7 will take a long time to be used in the production environment. So are there any alternatives to the current PHP5?
According to research, there is a very convenient alternative:

<code class="php">// 获取user参数的值(如果为空，则用&#39;nobody&#39;)
$username = @$_GET[&#39;user&#39;] ?: &#39;nobody&#39;;
// 等价于:
$username = isset($_GET[&#39;user&#39;]) ? $_GET[&#39;user&#39;] : &#39;nobody&#39;;
</code>

-- Run this code: https://3v4l.org/aDUW8

I looked at it with my eyes wide open. It was similar to the previous PHP7 example, mainly replacing ?? with ?: . What the hell is this? In fact, this is the omission pattern of the (expr1) ? (expr2) : (expr3) expression:

<p>表达式 (expr1) ? (expr2) : (expr3) 在 expr1 求值为 TRUE 时的值为 expr2，在 expr1 求值为 FALSE 时的值为 expr3。<br/>
自 PHP 5.3 起，可以省略三元运算符中间那部分。表达式 expr1 ?: expr3 在 expr1 求值为 TRUE 时返回 expr1，否则返回 expr3。<br/>
-- http://php.net/manual/zh/language.operators.comparison.php</p>

Of course, this alternative is not perfect - if there is no $_GET in 'user', there will be an Notice: Undefined index: user error, so you need to use @ to suppress this error, or turn off E_NOTICE mistake.

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