I had a strange, as it seems to me now, problem. Before that, I did not work with databases, so I ask for help from specialists!
I wrote scripts for registration, created a database for users, and created columns there, id: auto, (username, email, password): VARCHAR(255) and a couple of columns with date. Requests post work fine, the problem is only that "(username, email, password)" are created as null and I can not understand what the actual problem is, because the encoding is UTF8.
Here are the screenshots, because I think the code makes no sense to write here.





username=aaa&email=bbb&password=ccc? - IgorTextand the server takesApplication/json- Pavel Igorevich