問題描述

這個(gè)問題困擾了我很久，一直找不到答案...注意到它被 Zend 框架開發(fā)人員大量使用，

This is a question that is bugging me for a long time and can't find any answer... Noticed it's used quite a lot by Zend Framework Developers,

以下兩個(gè)if"語句有什么區(qū)別?:

What is the difference between following 2 "if" statements? :

if (null === $this->user) { ... }

if ($this->user === null) { ... }

對(duì)我來說第一個(gè)看起來有點(diǎn)奇怪;]

To me the first one looks kinda odd ;]

感謝您的回答.

推薦答案

這與您的腳本的工作方式?jīng)]有區(qū)別，它只是一個(gè)編碼標(biāo)準(zhǔn)，一個(gè)建議

This is not a difference for the way your script works, it's just a coding standard, a recommendation

推薦使用這種方式的原因:

The reason why it is recommended to use it this way:

if (null == $this->user)

的事實(shí)是，如果你輸入錯(cuò)誤并寫 = 而不是 == 你會(huì)得到一個(gè)錯(cuò)誤，而

is the fact that if you mistype and write = instead of == you will get an error, while

($this->user = null)

代替

($this->user == null)

有效但會(huì)導(dǎo)致奇怪的錯(cuò)誤(賦值和最終值被評(píng)估為 bool 而不是比較)

works but causes weird bugs (assignment and the final value is evaluated as bool instead of comparison)

我猜它只是作為一種習(xí)慣擴(kuò)展到嚴(yán)格比較運(yùn)算符 (===)

and I guess it just extended as a habit to the strict comparison operator (===)

更新:因?yàn)槲野l(fā)現(xiàn)即使在我發(fā)布答案 3 年后該線程上仍有一些活動(dòng)，所以我想我會(huì)添加一些我忘記提及的內(nèi)容.這種類型的符號(hào)稱為 yoda conditions，您可以在 this 上閱讀有關(guān)它的更多信息例如維基百科頁面.

Update: since I see that there's still some activity on this thread even 3 years after I posted the answer I figured I would add something I forgot to mention. This type of notation is known as yoda conditions, you can read more about it on this wikipedia page for example.

這篇關(guān)于PHP - if 語句中的倒序的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！