登录页面不工作

我正在web服务器中上传我的网站，但我的登录不起作用。在localhost上一切都很好。现在，当我输入正确的用户名和密码时，它会告诉我用户名/密码组合不正确。当我输入错误的密码时，它会再次输入。其他验证工作正常，我的代码：登录.php

<?php 
include 'core/init.php'; 
if(empty($_POST) === false) {
    $username= $_POST['username'];
    $password = $_POST['password'];
if(empty($username)=== true || empty($password) === true ) {
    $errors[] = 'You need to enter a username & password';
}else if (user_exists($username) === false) {
    $errors[]='We cant find that username.Have you registered?';
}else if (user_active($username) === false) {
    $errors[]='You havent activated your account!';
}else {
    if(strlen($password) > 32) {
        $errors[] = 'Password too long';
    }
    $login = login($username, $password);
    if($login === false) {
        $errors[] = 'That username/password combination is incorrect';
    }else{
$_SESSION['user_id'] = $login;
header('Location: index2.php');
exit();
    }       
    }
}else {
$errors[] = 'No data received';
    }
    include 'overall/headerr.php';
if (empty($errors)=== false) {
    ?>
    <h2>We tried to log you in, but...</h2>
    <?php
    echo output_errors($errors);
}
?>

users.php

function login($username, $password){
        $user_id = user_id_from_username($username);
        $username = sanitize($username);
        $password = md5($password);
        return (mysql_result(mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `username`='$username' AND `password`='$password'"), 0)==1) ? $user_id : false;
}
function logged_in() {
    return (isset($_SESSION['user_id'])) ? true : false;
}
function user_exists($username) {
    $username = sanitize($username);
  return (mysql_result( mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `username` ='$username' "), 0) == 1) ? true : false;
  }
function user_active($username) {
    $username = sanitize($username);
    return(mysql_result(mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `username` = '$username' AND `active` =1"), 0) == 1) ? true : false;
}
function user_id_from_username($username) {
    $username = sanitize($username);
    return mysql_result(mysql_query("SELECT `user_id` FROM `users` WHERE `username` = '$username'"), 0, 'user_id');
}
general.php
<?php 
function sanitize($data) {
    return mysql_real_escape_string($data);
}
function output_errors($errors) { 
$output = array();
foreach($errors as $error) {
$output[] = '<li>'. $error .'</li>';
}
return '<ul>' . implode ('', $output) . '</ul>';
}
?>

init.php

<?php 
session_start();
require 'database/connect.php';
require 'functions/general.php';
require 'functions/users.php';
$errors = array() ; 
?>