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Mateusz Jasiński
Mateusz Jasiński

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Coding DIY pt.1 - Login form in PHP

Hi everyone. Today I'll show you how to write a basic login form in PHP


In this tutorial I assume you are familiar with

  1. HTML
  2. Using code editor of your choice (Saving, creating and deleting files)

And here you will learn

  1. Basic PHP and SQL syntax
  2. How to run PHP and SQL on Windows machine
  3. How to protect your PHP code from SQL injection attack

So, when we have this covered. Let's start coding

1. Set up your environment

You can't run PHP code in your browser, as it's server-side language. You need server with an interpreter on it

So, to run PHP you must:

  1. Buy (Or find for free) a hosting with PHP running on it, to upload your files
  2. Set up your own server with PHP on your computer

Here, we will go with the second option. I know, it can sound scary but don't worry. It's not that hard

Disclaimer: I am using Windows in this tutorial. As far as I know for MacOS steps will be similar, but for Linux there is also other option by manually installing every package (If you would like a tutorial, leave some note in comments)

Let's go, we will be using XAMPP package here - first download installer from their official website

Then, run it.

If you have another partition on your machine (like Windows C and D) it's better to install it on one, without OS files.

Why? Sometimes it may cause problems, with permissions for example.

After it's completed go to xampp folder.

There will be a lot of folders

Folders in  raw `xampp` endraw  directory

But now, we focus only on one - htdocs

That's the folder where you store every PHP script you want to run

Get inside and delete everything, then create a new folder named login-form

There, we will store source code, for this project.

Inside it, create new file named index.php and open it in code editor

Inside it, write this simple code

 echo "server works";
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Okay, our test code is prepared - it will just display server works in browser

To run it, we need to start our server. In xampp folder find xampp-control.exe app. And run it, you will see something like this

XAMPP controll panell

Now, click start button next to Apache and MySQL

Why? Apache is our PHP server and MySQL is our database (DBMS to be more precise)

Ok, last thing to do, is to visit http://localhost/login-form

You can either click this link, or write it manually as an URL

If you see a page like this

Proper script execution

Everything works fine

So, environment is set up, let's start coding

2. Creating form on front-end

I hope you know HTML enough to create forms.

We need a form with 2 input fields and submit

Like this

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Login form</title>
    <form action="login.php" method="post">
        <input type="text" name="login" placeholder="Enter login:">
        <input type="password" name="password" placeholder="Enter password:">
        <button type="submit">Log in</button>
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Save it into index.php

In <form> tag we have 2 important parameters

  1. action - this indicates where will our request be sent to. Here we specify file in our directory named login.php. If you leave it blank, request will be sent to the same file, that it was sent from (so index.php in this case)

  2. method - It specifies what HTTP method will be used. There are 9 of them (According to MDN). We will be using POST method here, but you can also see GET method used there

So, front-end is ready. You can add some CSS if you want

Let's start coding login mechanism now

3. Basic data validation

First, to even start writing PHP code, we need to create "space" for it, by writing language tags first

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This tells our interpreter that, here we will have PHP code to execute. We don't need to close it, as in login.php file we will have only PHP code

Start with checking, if we even got necessary data

if(!isset($_POST['login']) || !isset($_POST['password'])){
    header("Location: index.php");
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If we anted to read this code aloud, it says something like this

If login field in _POST array isn't set, or if password field in _POST array isn't set, send user to index.php and finish script execution

Using $ we declare variable. And $_POST is one of many superglobals - variables available everywhere in the project

So, we are sure that we have every necessary data. Now save them to new variables and filter them.

$login = htmlentities($_POST['login'], ENT_QUOTES, "UTF-8");
$password = htmlentities($_POST['password'], ENT_QUOTES, "UTF-8");
$isValid = True
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If there are any quotes in this variables, PHP won't interpret them as quotes ending a string, but a part of string. That's how we can protect our code from code injection -> where an attacker places malicious code in a website to execute

I also declared isValid variable - this will be our flag to check if both login and password passed every validation stage

Now, check if none of them is empty. If one of them is, set the flag to false, and create an error message

if(empty($login) || empty($password)){
    $isValid = false;
    $errorMsg = "Login and/or Password can't be empty";
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Let's also implement some rules -> password must be minimum 8 characters long

if(strlen($password) < 8){
    $isValid = false;
    $errorMsg = "Password must have at least 8 characters" ;
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First part is over, now let's do database connected stuff.

4. Database creation

Start with visiting http://localhost/phpmyadmin
Then find new on the sidenav

You should see window like this

phpMyAdmin - database creation

In an input field, input db name - for me it's login-form

And choose proper encoding for your language - for me it's utf8_polish_ci

Then press create

Second thing we need to do is to create a table

When database is created you will see page with this form

Table creation form

Name this table users and choose 3 columns

Now, the last part of creating a table

Naming columns

Name them id, login, password

For id, INT type is good to go.

But rest, should be type TEXT.

Also, every table needs it's primary key.
Check A_I box - This will add sequentially new id's and we won't have to remember (or code) which was the last id in this table

This will also automatically make id our primary key

In the end, it should look like this

Final look of our table

Then click save button, and that's it. Table created.

Now if we want to log in we need to create a user

I used this credentials
Username: User1
Password: MyCoolPassword

Before even writing the query, we need to cover one topic - hashing

You don't store passwords in plaintext in database.


Because if someone would hack your website and dump it (got db saved into the file with queries, allowing to reproduce it with all data), they can get your users password and hack their other accounts (some people reuse easy to guess passwords)

That's where hashing comes in handy. Hashing is method of one-way encryption. You give password to hashing function and it returns you string of bytes. ex.

MyCoolPassword hased in md5 algorithm will be 539eb79895088d1fc956abec4a07cbc4

Disclaimer: MD5 algorithm shouldn't be used in applications today. It's not safe, there are better algorithms

Let's hash password with default PHP hash

echo  password_hash("MyCoolPassword", PASSWORD_DEFAULT);
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After running this code in some other file, I've got $2y$10$wqobdTP/8yHOfhbA0LrIB.nXTAxWixQTvVBqQKwQC6383iqSJoOGq- That's our hash

To insert data into SQL table we use INSERT query

First, specify where you want to insert query

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Then we want our values

For id - null
For login - User1
For password - $2y$10$wqobdTP/8yHOfhbA0LrIB.nXTAxWixQTvVBqQKwQC6383iqSJoOGq

So, let's finish our query

INSERT INTO `users` VALUES(null, "User1", "$2y$10$wqobdTP/8yHOfhbA0LrIB.nXTAxWixQTvVBqQKwQC6383iqSJoOGq");
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Input this Query into big text area in SQL tab of users table in our DB

If we look at our database now, you can see something strange.

Query result

We inserted null as value for id but now we have 1, why?

That's the magic of AUTO_INCREMENT - it will automatically assign value to this field and it will be next integer

Ok, everything done. Now go back to your code editor

5. Creating DB connection

Before connecting: If passed data hadn't met previous requirements, why should we even bother to connect to the database if we know that login won't be successful?

Let's create if statement that will finish execution if $isValid is false

    echo $errorMsg;
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Ok, we are ready to start creating our DB connection script

To connect to database and run queries we will use PDO and prepared statements. You can learn more about them in this article

Start with adding 3 variables to the beginning of login.php

$db_dsn = "mysql:dbname=login-form;host=localhost";
$db_user = "root";
$db_pass = "";
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I think you can deduct what is in $db_user and $db_pass
But, what is $db_dsn?

DSN stands for Data Source Name. It's a string that tells PDO what driver should it use (what DBMS are we connecting to), what database (login-form) and where is it located (localhost)

Now, we need another variable - now for DB connection, this can be placed at the end

$db = new PDO($db_dsn, $db_user, $db_pass);
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What it does? It creates for us new object of PDO class. It makes connecting to database possible

We have connection, so let's write some SQL

$sql = "SELECT * FROM `users` WHERE login=?";
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In SQL there is a rule, that every keyword is capitalized.
* in SQL means all

So, to get better understanding - Read this SQL aloud

Select all from users table where login equals ?

Why ? -> it's sign for PDO, that here is the place to place bind data

Now, let's execute this query and get it's results

$stmt = $db->prepare($sql);

$result = $stmt->fetch(PDO::FETCH_ASSOC);
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After we run the query, we need to know whether user even exists in database

    $isValid = false;
    $errorMsg = "Incorrect username or password";
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Now, let's validate the password, this should be done only if user exists, so let's put it into else for previous if

    if(!password_verify($password, $result['password'])){
        $isValid = false;
        $errorMsg = 'Incorrect username or password';
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Why do we use function instead of == or ===? Sometimes if we wanted to hash password again, we may get slightly different hash.

But when we use password_verify(), we can be sure that password will be properly compared to hash

That's every comparison we need, let's check if $isValid flag is true.

Redirect user to another page with some content accessible after successful login or display error after failed one.

    echo $errorMsg;

header("Location: secret.php");
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That's content visible only after login in my case. You can add there whatever you want

<!DOCTYPE html>
<html lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Secret website</title>
    <h1>Welcome on secret page</h1>
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After successful login we see

Secret page

Now, we have one thing left

6. Displaying Errors

Ok, So we have everything ready and good to go, but wouldn't we like to have error messages in index.php and not login.php?

That's the task for $_SESSION superglobal. It allows us to save data into session storage and reuse them on other pages in our website. We can often find stuff like tokens in there

If we want to use it, we need to signalize it. Start the session at the beginning of every file.

In index.php and secret.php


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In login.php

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(of course after <?php tag)

Now, we need to edit every if where we are checking !$isValid condition, save $errorMsg value into session variable with the same name and, redirect to index.php

    $_SESSION['errorMsg'] = $errorMsg;
    header("Location: index.php");
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Now, go back to index.php and below the form we need to handle and show error

Check if errorMsg variable is even set, then display and unset it

            echo $_SESSION['errorMsg'];
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We also need another validation. If user that accesses secret.php should be able to see it

In login.php let's add another session variable called isLoged and set it right over the redirection

$_SESSION['isLoged'] = true;
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Last, but not least. Check it in secret.php

    header("Location: index.php");
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Now, without logging in, try to visit http://localhost/login-form/index.php

We can't do it now. That's what we wanted.
Only authenticated users can access that page

And that's it. We have nice, and working login form


So now, let's sum up what you know

  1. How to build login script
  2. Basic PHP and SQL syntax
  3. How to validate and hash passwords
  4. How to use XAMPP and PHPMyAdmin
  5. How to use session variables in PHP

That's the end of this article. I hope you learnt something useful today. Leave your feedback and ideas for another posts in comments. I encourage you to check my other posts like tryhackme write-ups or PHP-themed articles

Code for this is available on my Github, so you can add your feedback there too, or just compare if something isn't working

See you in next articles

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