目录一、Null的本质：从“缺失值”到“设计哲学”1.1 Null的定义与历史渊源1.2 Null引发的“灾难性后果”二、5大Null运算符：对比与实战运算符1：Null合并运算符（??）运算符2：空条件
so in my php I have something like this $_SESSION[\'opened\'] = true; But It will no开发者_StackOverflowt be set to true until user will perform some actions with some other html\\php pages
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