Código PHP:
Notice: Undefined variable: mysqli in /var/www/vhosts/jarbbo.com/httpdocs/register.php on line 261 Fatal error: Call to a member function query() on a non-object in /var/www/vhosts/jarbbo.com/httpdocs/register.php on line 261
Código PHP:
$nick_check = $mysqli->query("SELECT id FROM users WHERE username = '".$_POST['username']."'");