Aquí está mi código. Por alguna razón, si envío el formulario sin colocar ni contraseñas, todavía crea la entrada de la base de datos. Hay algunos comentarios dispersos en todo el código, pero el código es bastante sencillo. ¿Algunas ideas

<?php
//signup.php
include 'connect.php';
include 'header.php';

echo '<h3>Sign up</h3>';

if($_SERVER['REQUEST_METHOD'] != 'POST')
{
    /*The form hasn't been posted yet, display it
      note that the action="" will cause the form to post to the same page it is on */
    echo '<form method="post" action="">
        Username: <input type="text" name="user_name" /><br />
        Password: <input type="password" name="user_pass" /><br />
        Password again: <input type="password" name="user_pass_check" /><br />
        E-mail: <input type="email" name="user_email" /><br />
        <input type="submit" value="Add category" />
        </form>';
}
else
{
    /* so, the form has been posted, we'll process the data in three steps:
        1.  Check the data
        2.  Let the user refill the wrong fields (if necessary)
        3.  Save the data
    */
    $errors = array(); /* declare the array for later use */

    if(isset($_POST['user_name']))
    {
        //the user name exists
        if(!ctype_alnum($_POST['user_name']))
        {
            $errors[] = 'The username can only contain letters and digits.';
        }
        if(strlen($_POST['user_name']) > 30)
        {
            $errors[] = 'The username cannot be longer than 30 characters.';
        }
    }
    else
    {
        $errors[] = 'The username field must not be empty.';
    }

    if(isset($_POST['user_pass']))
    {
        if($_POST['user_pass'] != $_POST['user_pass_check'])
        {
            $errors[] = 'The two passwords did not match.';
        }
    }
    else
    {
        $errors[] = 'The password field cannot be empty.';
    }

    if(!empty($errors))
    {
        echo 'Uh-oh.. a couple of fields are not filled in correctly..';
        echo '<ul>';
        foreach($errors as $key => $value)
        {
            echo '<li>'.$value.'</li>';
        }
        echo '</ul>';
    }
    else
    {
        //the form has been posted without errors, so save it
        //notice the use of mysql_real_escape_string, keep everything safe.
        //also notice the sha1 function which hashes the password
        $sql = "INSERT INTO
                    users(user_name, user_pass, user_email, user_date, user_level)
                VALUES('" . mysql_real_escape_string($_POST['user_name']) . "',
                       '" . sha1($_POST['user_pass']) . "',
                       '" . mysql_real_escape_string($_POST['user_email']) . "',
                       NOW(),
                       0)";

        $result = mysql_query($sql);
        if(!$result)
        {
            //something went wrong, display the error
            echo 'Something went wrong while registering. Please try again later.';
            //echo mysql_error(); //debugging purposes, uncomment when needed
        }
        else
        {
            echo 'Successfully registered. You can now <a href="signin.php">sign in</a>
                     and start posting!';
        }
    }
}
include 'footer.php';
?>