One problem with singleton mode, static $db = null Why do only once

  
   为什么 结果只输出一次1111上面的static $db = null这里不是每执行一次get()就重新对$db付值为null了吗，求解下

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   为什么 结果只输出一次1111上面的static $db = null这里不是每执行一次get()就重新对$db付值为null了吗，求解下

Since the parameter is static, you are at the end of $db = new DatabaseConnection (); The second time you came in, the $db changed for the first time, so it's not null.
I don't believe you. Static to public, it should appear two times

The second $db is not re-assigned, only the first time it is initialized, refer to the Document link description

staticVariables have the same scope and local variables as the life cycle and global variables.

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