PHP database has no content, solution, super beginner. Ah

PHP database has no content, solution, super rookie .. Hi, I have made a message board. are you able to log on to a user, log out, register, view, and leave a message online? the database is also ready. among them, I have registered successfully, then log on .. But failed (the password is correct ...), There is no data in the database .. Ah. I don't understand. I want to send my php code to something I'm afraid of. (do you know you can upload compressed files? This makes it easier for me to know that PHP databases do not have any content .. Ah
I have made a message board. Are these functions available? are users logged on, logged out, registered, viewed, online messages, databases, and so on? among them, I have registered successfully, then log on .. But failed (the password is correct ...), There is no data in the database .. Ah. I don't understand. I want to send my php code to something I'm afraid of. (do you know you can upload compressed files? I 'd like to know more about this)

Forgive me for not having many points. I started the scdn activity last night. So if the score is less, there will certainly be many points when the score is richer. Try it together and help me solve it.

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You can post the code for registration.