Upgrade PHP to php7 The impact of X (I). The problem of session retention. Keep login

Why can't I log in , The main reason is session

A recent project is going to use php7.x, But the company servers are 5.5 and 5.6 Of , Customer needs are above all else , Upgrade the server to 7.2 I won't be able to log in later , The reason is that the login status cannot be maintained .
Because wechat login is used , You can't see the debugging information intuitively , Only various terminals can be added , And write log text
Last and last , Hematemesis is session No normal write , This causes the second refresh of the page session non-existent .
by the way , Insert a sentence , This is the case for session Write to the database , If session It should be unaffected in the form of a file .
That is to use this function session_set_save_handler,（ Although I have studied in a class , but phper But it belongs to a halfway monk , Forgive my poor words , I really don't know what the name of this form is , It doesn't affect the development and doesn't bother to check ）, stay read At the end of the function ,5.x How many? return false,7.x requirement return ‘’, Cause unable to write session
Not much to say Upper figure , Those who often do project deduction code should be able to see which source code this is .

Read the article carefully , This problem has been solved in the middle of the night , Finally, I read the first article and solved it