MySQL数据库检查

This is my code

$username = $_POST['user'];
$password = $_POST['pass'];

if (isset($_POST['user'])); {
    $db = mysqli_connect('localhost', 'root', '', 'db');
    if($query = mysqli_query($db, "SELECT `pass` FROM `accounts` WHERE `user` = '$username'")){
      while($row = mysqli_fetch_assoc($query)){
        $row['pass'] = $setpassword;
      }
      mysqli_free_result($query);
    }
}

What it currently does is from a form, retrive a username and password that the user has entered, take that username and find the row with that username and get the password from that row and set it as the variable $setpassword. Below is the code to check if the password matches the given username on the database.

if ($password=='') {
    $verify = 0;
}

if ($password!='') {
    if ($password!=$setpassword) {
        $verify = 1;
    }
    if ($password==$setpassword) {
        $verify = 2;
    }
}

If verify is... 0 - The Login Form Will appear as nothing has been entered. 1 - Incorrect Password will be displayed along with the login form. 2 - Correct Password will be displayed and the username will be assigned to a session variable.

I'm having a problem where a user can enter a username that doesnt exist and any password wether its in the database or not and it will be verified.

What can I do to check if the username doesn't exist on the database?

When you are accepting the user's registration query the database to see if it already exists.

$result = mysqli_query("SELECT * FROM accounts where `user` = $username");

if(mysql_num_rows($result) >0) // if there are any rows returned then the username exists
{ 
    //User Name already exists
}
else
{
   //User name doesn't exist, add user
}

I'm not sure this is where you are doing that. But to eliminate duplicates you can do it that way. Also, you can define the column user as unique. That way the SQL will not allow duplicate values.

Also this line:

$row['pass'] = $setpassword; //setting $row['pass'] to $setpasswords value.

This is reversed. You should be doing it the other way around.

 $setpassword = $row['pass']; //setting setpassword to $row['pass'] value.

Let me know if I need to clarify anything.

Try this:

$username = isset($_POST['user'])?$_POST['user']:''; // check if isset to avoid notice
$password = isset($_POST['pass'])?$_POST['pass']:'';
$verify = 0; 

if (!empty($username)) {
    $db = mysqli_connect('localhost', 'root', '', 'db');
    if($query = mysqli_query($db, "SELECT `pass` FROM `accounts` WHERE `user` = '$username'")) { 
    while($row = mysqli_fetch_assoc($query)){
       $setpassword = $row['pass'];
       break; // exit the loop once you found the password
    }
    mysqli_free_result($query);    
} 

if (isset($setpassword)) {
   $verify = 1;       
   if ($password == $setpassword) {
      $verify = 2;
   }
}

if (isset($_POST['user'])); { 

there is an extra semicolon in this line, making whole code not working

to do your verification, all you need is to retrieve the password and compare it with entred one:

$row = mysqli_fetch_assoc($query));
if ($row AND $row['pass'] == $password)
  $verify = 1;
}

note that $row could be ampty, so, you have to check it first

however, you can do both comparisons in the query, like this

"SELECT * FROM accounts where `user` = $username" AND `pass` = '$password';

However, your code suffers from 2 common problems.

1. It is better to save a hash instead of the plain password.

2. You should sanitize your data before adding it in the query

at least this way:

$username = mysqli_real_escape_string($db,$_POST['user']);