A few PHP basics, but there's no problem with the book.

A few PHP basics, but the book didn't find a problem.
See a lot of the framework inside some code with two underscores, such as __isset, __construct and so on, is not in front of the custom function can be added two underline? What is the effect of two underscores?
There is a _e, what is this??

These are not in basic programming, are they? I haven't seen it in the basic tutorial??

Everybody, teach me a lot.

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Search: Php Magic method
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Adding no underline has no practical effect, just let beginners do not name it, avoid the possibility of conflict with their own methods.
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The above two are magic methods, the following should be the framework of a custom method

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