将php代码从mysql更新为mysqli，未定义的变量，但在构造函数中定义

I downloaded this code from androidhive. It's login/registration code for android apps. Now the php code is from two years ago and so I was originally gettin depreciated errors and so I began updating the code to mysqli.So far when converting, I've been changing the mysql functions to mysqli, and if need be adjusting the parameters. I'm fairly new to php in general so I've been running into a few problems, the main one being that of isUserExisted function in DB_functions the variable db is undefined for the mysqli_query. I'm not sure why I'm getting this error because I've defined it in the constructor. I've looked at similar questions and the answers that fixed their problems were usually syntax errors, which when I compare this code, I don't seem to have. Any help would be greatly appreciated. Here's the DB_Functions.php and the DB_Connect.php respectively:I downloaded this code from androidhive. It's l