<?php
error_reporting (E_ALL & ~ E_NOTICE & ~ E_DEPRECATED);
$hostname = 'localhost';
$username = 'root';
$password = '12345';
$db = 'rifou';

$connection = new mysqli($hostname, $username, $password);
mysqli_select_db($connection, $db);
$nickname = mysqli_real_escape_string($connection, $_POST['nick']);
$password = mysqli_real_escape_string($connection, $_POST['senha']);
$select_user = "SELECT * FROM usuarios WHERE nick='$nickname' AND senha='$senha'";
$run_user = mysqli_query($con, $select_user);
$check_user = mysqli_num_rows($run_user);
if ($check_user > 0){
    echo "logado";
} else {
   echo "não logado";
}
?>

$connection = new mysqli($hostname, $username, $password);

$run_user = mysqli_query($con, $select_user);

$connection = new mysqli($hostname, $username, $password);
mysqli_select_db($connection, $db);
//Linhas omitidas
$run_user = mysqli_query($con, $select_user); //<--- deve $connection e não $con