add-user-form

<div id="add-user-form">
    <form method="post" autocomplete="on">
        <fieldset class="login-form">
            <label for="username">Username:</label>
            <input type="text" name="username" id="username" autofocus pattern="^[a-zA-Z][a-zA-Z0-9-_\.]{1,30}$">
        </fieldset>

        <fieldset class="login-form">
            <label for="password">Password:</label>
            <input type="password" name="password" id="password" autofocus required >
        </fieldset>

        <fieldset class="login-form">
            <label for="level">Privilegies:</label>
            <select name="level">
                <option value="1">Admin</option>
                <option value="2">User</option>
            </select>
        </fieldset>

        <fieldset class="login-form">
             <input class="read-more" type="submit" formnovalidate name="submit-add-user" id="submit-add-user" value="Add">
        </fieldset>

    <?php
    if(isset($_POST['submit-add-user'])) {
        require ('/connection.php');

        $username=$_POST['username'];
        $password=$_POST['password'];
        $level=$_POST['level'];

        $username = stripslashes($username);
        $password = stripslashes($password);
        $username = mysqli_real_escape_string($username);
        $password = mysqli_real_escape_string($password);
        $password = sha1($password);

        $default_avatar="obrazky/avatars/0.jpg";

        $sql="INSERT INTO user (username, password, privileges_level, avatar_url) VALUES ('$username', '$password', '$level', '$default_avatar')";
        $result=mysqli_query($connection,$sql);

        if (!$result) {
           echo "chyba";
        }
        else {
            header("Location: add-user-form.php");
            mysqli_close($connection);
        }
    }
    ?>
     </form>
</div>