新用户创建后mySQL访问被拒绝错误

I made some projects with using mysql and there was no problem. Today my friend show me how to create user in mysql by :

Create database cc;
use cc
CREATE USER 'testing'@'localhost' IDENTIFIED BY 'pass';
GRANT ALL ON cc.* TO 'testing'@'localhost';

Now I tried to use my project but I got following error :

Connection failed: Access denied for user ''@'localhost' to database 'login'

I deleted previously created user :

drop user 'testing'@'localhost';

I managed to delete that user, I checked user with :

select User from mysql.user;

testing account was gone but now I have 3 root accounts, I didn't understand why. Since I deleted that "testing" account, I tried my project again and I got the same error. Why I can't use default 'root' account anymore?

EDIT :

Error part of php code :

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "login";


// Error is in following line
$conn = mysqli_connect ( $servername, $username, $password, $dbname );

I also tried this :

GRANT ALL ON login.* TO 'root'@'localhost';

show grants; 

Result is :

GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION
GRANT ALL PRIVILEGES ON login.* TO 'root'@'localhost'

It seems like 'root' has access to everything. Why am I getting error?

EDIT 2:

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "login";
$username = $_POST['username'];
$pass = $_POST['password'];

// Create connection
$conn = mysqli_connect ( $servername, $username, $password, $dbname );
// Check connection
if (! $conn) {
    die ( "Connection failed: " . mysqli_connect_error () );
}

Your over riding the previously set $username variable with the $_POST['username'] data which appears to be empty. So mysqli_connect attempts to connect to the database with a blank username.

To correct this, rename the $username variable to something else which doesn't conflict.

Example:

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "login";
$username_post = $_POST['username'];
$pass = $_POST['password'];

// Create connection
$conn = mysqli_connect ( $servername, $username, $password, $dbname );
// Check connection
if (! $conn) {
     die ( "Connection failed: " . mysqli_connect_error () ); 
}