Simple signup/ login example by Miro Balearski

<?php

/****
Db schema

create table users (
    id int(10) not null primary key auto_increment,
    username varchar(255) not null unique,
    password varchar(255) not null,
    email varchar(255) not null default ''
);

****/


function get_dblink(){
    return  mysqli_connect("localhost","db_username","db_password","db_name");
}

function register_form(){?>
    <form method="post" >
        Username :<input type = "text" name="username"><br/>
        Passwod: <input type = "password" name="pass"> <br/>
        Retype Passwod:<input type = "password" name="pass2"><br/>
        <input type="hidden" name="action" value="register"><br/>
        <input type="submit" value="Register"><br/>
    </form>
    <?php
}

function login_form(){?>
    <form method="post">
        Username :<input type = "text" name="username"><br/>
        Passwod: <input type = "password" name="pass"><br/>
        <input type="hidden" name="action" value="login"><br/>
        <input type="submit" value="Login"><br/>
    </form>
    <?php
}

function logout_form($username){ ?>
    <form method="post">
        <input type="hidden" name="action" value="logout">
        Logout <?=$username?> <input type="submit" value="Logout">
    </form>
    <?php
}


/**************************************************************/


#Validators :

function validate_username_size_and_chars($username){

    if(preg_match('/^[a-zA-Z0-9]{5,}$/', $username)) {
        return true;
    }else{
        throw new Exception("The username should contain a-zA-Z0-9 and should be long 5 chars or more ") ;
    }
}

function validate_password_size_and_chars($pass){

    if(preg_match('/^[a-zA-Z0-9]{5,}$/', $pass)) {
        return true;
    }else{
        throw new Exception("The password should contain a-zA-Z0-9 and should be long 5 chars or more ");
    }
}
function validate_password_dont_match_the_username($pass, $username){

    if($pass===$username){
        throw new Exception("The password can not be the same as the username");
    }else{
        return true;
    }
}

function validate_registration_passwords_match($pass, $pass2){

    if($pass===$pass2){
        return true;
    }
    throw new Exception("The passwords don't match");
}

function validate_username_available($username){

    if(get_user_id_by_username($username)){
        throw new Exception("The username \"$username\" is already registered");
    }
    return true;
}

# DB functions ################################################

function authenticate_user($username, $password){

    $username= mysqli_real_escape_string(get_dblink(),$username);

    $qry = "select id, username, password from users where username='$username' ";
    $res= mysqli_query(get_dblink(), $qry);
    $rr= mysqli_fetch_assoc($res);

    return password_verify($password, $rr['password']);
}

# Register user

function add_user_to_the_database($username,$pass){

    $dblink=get_dblink();

    $hashed_pass = password_hash($pass, PASSWORD_BCRYPT);
    $username= mysqli_real_escape_string(get_dblink(),$username);
    $qry = "insert into users(username, password) values ('$username', '$hashed_pass')";

    mysqli_query( $dblink ,$qry);
    if (mysqli_error($dblink)){
        throw new Exception("Error inserting into the DB ". mysqli_error($dblink));
    }
}

function register_user($username, $pass, $pass2){

    try{
        validate_username_size_and_chars($username);
        validate_username_available($username);
        validate_password_dont_match_the_username($pass,$username);
        validate_password_size_and_chars($pass);
        validate_registration_passwords_match($pass, $pass2);

            # and finally
        add_user_to_the_database($username,$pass);

            # todo email validation
            # or
            # automaticaliy login the new user;

        login_user($username, $pass);

    } catch (Exception $e){

        print_r("<div style='color:red'>".$e->getMessage()."</div>");
    }
}

function get_user_id_by_username($username){

    $username= mysqli_real_escape_string(get_dblink(),$username);
    $qry = "select id  from users where username='$username' ";
    $res= mysqli_query(get_dblink(), $qry);
    $rr= mysqli_fetch_assoc($res);
    return $rr['id'];
}

function login_user($username, $password){

    if(authenticate_user($username, $password)){
        $_SESSION['valid_user_id'] = get_user_id_by_username($username);
        $_SESSION['username']=$username;

    }else{
        print_r("<div style='color:red'>Invalid username or password</div>");
    }
}

function loggedin_user(){
    return isset($_SESSION['valid_user_id']) ? $_SESSION['valid_user_id'] : 0;
}

function logout_user(){
    session_destroy();
    unset($_SESSION);
}

/*****************************************************************************/

session_start();

# handle http post requests

if(isset($_POST['action']))
    switch($_POST['action']){

        case "login":
            login_user($_POST['username'], $_POST['pass']);
        break;
        case "logout":
            logout_user();
        break;
        case "register":
            register_user($_POST['username'], $_POST['pass'], $_POST['pass2']);
        break;
    }

# application

if(!loggedin_user()){

    # show_some_content_for_NOT_logged_in_users_here();

    echo "<h1>You are not logged in. Please Login or Register</h1>\n";

    echo "<H2>Login:</H2>\n";
    login_form();

    echo "<br><H2>Register:</H2>\n";
    register_form();

    # not registered users exprience stops here
    exit(0);
}


# This content will be available only for logged in users

echo "<h1>You are logged in as \"$_SESSION[username]\" </h1>";
logout_form($_SESSION['username']);

# show_content_for_logged_in_users_only();