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">
        Passwod: <input type = "password" name="pass">
        Retype Passwod:<input type = "password" name="pass2">
        <input type="hidden" name="action" value="register">
        <input type="submit" value="Login">
    </form>
    <?php
}

function login_form(){

    ?>
    <form method="post">
        Username :<input type = "text" name="username">
        Passwod: <input type = "password" name="pass">
        <input type="hidden" name="action" value="login">
        <input type="submit" value="Login">
    </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($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($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 registration_passwords_match($pass, $pass2){

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


# 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){

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


    $dblink=get_dblink();
    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($username);
        validate_password_dont_match_the_username($pass,$username);
        validate_password($pass);

        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 login_user($username, $password){

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

    }
}

function loggedin_user(){

    return $_SESSION['valid_user_id'];

}

function logout_user(){

    session_destroy();
}

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

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()){

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

}else{
    echo "<h1>You are not logged in. Please Login or Register</h1>";
    echo "<br><H2>Register:</H2>";

    register_form();

    echo "<br><H2>Login:</H2>";
    login_form();
}