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php-如何检查一个变量是否被设置，并且值为NULL

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Release： 2016-06-02 11:32:00
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php编程语言

最近有一个小问题，就是如何在php中检查一个变量是否被设置并且值为null，看看大家都有什么好的解决方案？谢谢~~

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