What is the cause of these errors

Question

I',m trying to make a registration system here's the webpage displaying the error :
https://naijalife.000webhostapp.com/good.php
And the code :
https://code.sololearn.com/ww724T97Rsz2/?ref=app

Answer 1

FIX 1:
$username is undefined when not isset($_POST['username']).

You probably want the $sql calculation in that if-statement so it doesn't get hit when $_POST['username'] isn't set.

Moving the $sql calculation should fix the "Undefined variable: username" error.

FIX 2:
Also, I think you should also change $con to $mysqli in this line:
$con = mysqli_connect($DATABASE_HOST, $DATABASE_USER, $DATABASE_PASS, $DATABASE_NAME);

Renaming $con to $mysqli should fix some of the "Undefined variable: mysqli" messages.

There are other error messages but you should fix those 2 first.

After that gets working without any errors, you should look into prepared statements. real_escape_string and your escaping efforts will be simpler and more secure with prepared statements.

Answer 2

Start with adjust:
1) You use $mysli as var but your connection is referenced by $con var then you have to use it

2) $username is setted only if request type is post and contains an username parameter but you use it anyway and its bad

3) you dont check myslqli codes for errors