h

<html>
    <head>
        <title>My first PHP website</title>
    </head>
    <body>
        <h2>Registration Page</h2>
        <a href="index.php">Click here to go back</a><br/><br/>
        <form action="register.php" method="post">
            Enter Username: <input type="text" name="username" required="required"/> <br/>
            Enter Password: <input type="password" name="password" required="required" /> <br/>
            <input type="submit" value="Register"/>
        </form>
    </body>
</html>

<?php
if($_SERVER["REQUEST_METHOD"] == "POST"){
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);
    $bool = true;
    mysql_connect("localhost", "root","1234") or die(mysql_error()); //Connect to server
    mysql_select_db("first_db") or die("Cannot connect to database"); //Connect to database
    $query = mysql_query("Select * from users"); //Query the users table
    while($row = mysql_fetch_array($query)) //display all rows from query
    {
        $table_users = $row['username']; // the first username row is passed on to $table_users, and so on until the query is finished
        if($username == $table_users) // checks if there are any matching fields
        {
            $bool = false; // sets bool to false
            Print '<script>alert("Username has been taken!");</script>'; //Prompts the user
            Print '<script>window.location.assign("register.php");</script>'; // redirects to register.php
        }
    }
    if($bool) // checks if bool is true
    {
        mysql_query("INSERT INTO users (username, password) VALUES ('$username','$password')"); //Inserts the value to table users
        Print '<script>alert("Successfully Registered!");</script>'; // Prompts the user
        Print '<script>window.location.assign("register.php");</script>'; // redirects to register.php
    }
}
?>