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Home Backend Development PHP Tutorial PHP中的null合攏運算符

PHP中的null合攏運算符

Jun 13, 2016 pm 12:28 PM
nobody php user

PHP中的null合并運算符

project: blogtarget: null-coalesce-operator-in-php.mddate: 2015-12-30status: publishtags:    - Null Coalesce    - PHPcategories:    - PHP

null合并運算符是一個好東西,有了它我們就能很方便的獲取一個參數,并能在其為空的情況下提供一個默認值。比如在js中可以用||來搞:

function setSomething(a){    a = a || 'some-default-value';    // ...}

而在PHP中,可惜PHP的||總是返回truefalse,無法這樣來搞。
PHP7才正式加入了??這個運算符:

// 獲取user參數的值(如果為空,則用'nobody')$username = $_GET['user'] ?? 'nobody';// 等價于:$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

PHP7 估計還有很長時間才能用到生產環(huán)境中,那在目前的PHP5中有沒有替代方案呢?
據研究,完全有個非常便捷的替代方案:

// 獲取user參數的值(如果為空,則用'nobody')$username = @$_GET['user'] ?: 'nobody';// 等價于:$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

-- 運行此代碼: https://3v4l.org/aDUW8

瞪大了眼睛看,跟前面的PHP7的例子差不多,主要是把 ?? 替換為了 ?: 。 這個是個什么鬼呢?其實這就是 (expr1) ? (expr2) : (expr3) 表達式的省略模式:

表達式 (expr1) ? (expr2) : (expr3) 在 expr1 求值為 TRUE 時的值為 expr2,在 expr1 求值為 FALSE 時的值為 expr3。
自 PHP 5.3 起,可以省略三元運算符中間那部分。表達式 expr1 ?: expr3 在 expr1 求值為 TRUE 時返回 expr1,否則返回 expr3。
-- http://php.net/manual/zh/language.operators.comparison.php

當然,這個替代方案也不是完美的 —— 如果 $_GET 中沒有 'user',會有一條 Notice: Undefined index: user 的錯誤,所以需要用@來抑制這個錯誤,或者關閉 E_NOTICE 的錯誤。

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