在PHP中首先使用null有什么好处?

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可能重复: 为什么一些有经验的程序员用变量前的值写比较?

Possible Duplicate: Why do some experienced programmers write comparisons with the value before the variable?

我对此感到很好奇:在我研究过的大多数框架/开源项目中，我经常看到这样的代码...

I am just curious about this: in most frameworks/opensource projects I have studied, I often seen code like this...

<?php if (null === self::$_instance) { self::$_instance = new self(); }

特别是这一行...

if (null === self::$_instance) {

为什么在if语句的第一个参数中使用null而不是相反?...

Why use null in the first argument of the if statement instead of the other way around?...

if (self::$_instance === null) {

我意识到可能没有性能提升或类似的提升.这只是偏好还是我忽略的某种编码标准?

I realize there is probably no performance increase or anything like that. Is this just a preference or is it some kind of coding standard I have overlooked?

推荐答案

它可以防止您意外地将值分配给变量，尤其是在仅使用松散类型比较(==)时:

It prevents you from accidentally assigning the value to a variable, especially when only using loose type comparison (==):

if (self::$_instance = NULL) { … } // WHOOPS!, self::$_instance is now NULL

这种条件类型通常称为 yoda条件.在性能方面没有区别，两条语句是等效的.

This style of conditions is often called yoda conditions. Performance wise there is no difference, both statements are equivalent.