Cannot insert data into MySQL Database

So I'm having kind of a problem here. I'm trying to do something very basic, which is store data via Apache localhost website into a MySQL Database. After I send the data on my website, it's not stored into the database. I will post my code first and then give you some more info:

Code in header:

<?php
$link=mysqli_connect("localhost","root","mypassword","database") or die("Couldn't connect to DB");
?>

Code:

<?php include ( "./inc/header.inc.php" ); ?>
<?php
$reg = @$_POST['reg'];
// declaring variables to prevent errors
$fn = ""; // First Name
$ln = ""; // Last Name
$un = ""; // Username
$em = ""; // Email
$emc = ""; // Emailcheck
$pswd = ""; // Password
$pswdc = ""; // Passwordcheck
$d = ""; // Signup Date
$u_check = ""; // Check if username exists
// registration form
$fn = strip_tags(@$_POST['fname']);
$ln = strip_tags(@$_POST['lname']);
$un = strip_tags(@$_POST['username']);
$em = strip_tags(@$_POST['email']);
$emc = strip_tags(@$_POST['emailcheck']);
$pswd = strip_tags(@$_POST['password']);
$pswdc = strip_tags(@$_POST['passwordcheck']);
date_default_timezone_set('Europe/Berlin');
$d = date("Y-M-D"); // Year - Month - Day

if ($reg) {
    // Check if emails are equal
    if ($em==$emc) {
        // Check if user already exists
        $u_check = mysqli_query($link, "SELECT username FROM users WHERE username= '$un'");
        // Count the amount of rows where username = $un
        $check = mysqli_num_rows($u_check);
        if ($check == 0) {
            // Check all of the fields have been filled in
            if ($fn&&$ln&&$un&&$em&&$emc&&$pswd&&$pswdc) {
                // Check that passwords match
                if ($pswd==$pswdc) {
                    // Check the max length of username/first name/last name does not exeed 25 characters
                    if (strlen($un)>25||strlen($fn)>25||strlen($ln)>25) {
                        echo "The maximum limit for username/first name/last name is 25 characters!";
                    } else {
                        // Check the max length of password does not exeed 25 characters and is not less than 5 characters
                        if (strlen($pswd)>30||strlen($pswd)<5) {
                            echo "Your password must be between 5 and 30 characters long. So should your ######!";
                        } else {
                            // encrypt password and passwordcheck using md5 before sending to database
                            $pswd = md5($pswd);
                            $pswdc = md5($pswdc);
                            $query = mysqli_query($link, "INSERT INTO users (id, username, first_name, last_name, email, password, sign_up_date, activated) VALUES ('','$un','$fn','$ln','$em','$pswd','$d','0')");
                            die("<h2>Welcome to #######</h2>Login to your account to get ######!");
                        }
                    }
                } else {
                    echo "Your passwords don't match.";
                }
            } else {
                echo "Please fill in all of the fields.";
            }
        } else {
            echo "This Username already exists.";
        }
    } else {
        echo "Your Emails don't match.";
    }
}
?>
        <div id="wrapper2">
            <table>
                <tr>
                    <td width="60%" valign="top">
                        <h2>Join ####### today!</h2>
                    </td>
                    <td width="40%" valign="top">
                        <h2>Sign up below.</h2>
                        <form action="#" method="POST">
                            <input type="text" name="fname" size="25" placeholder="First Name" /><br /><br />
                            <input type="text" name="lname" size="25" placeholder="Last Name" /><br /><br />
                            <input type="text" name="username" size="25" placeholder="User Name" /><br /><br />
                            <input type="text" name="email" size="25" placeholder="E-Mail" /><br /><br />
                            <input type="text" name="emailcheck" size="25" placeholder="E-Mail Check" /><br /><br />
                            <input type="text" name="password" size="25" placeholder="Password" /><br /><br />
                            <input type="text" name="passwordcheck" size="25" placeholder="Password Check" /><br /><br />
                            <input type="submit" name="reg" value="Sign Up!">
                        </form>
                    </td>
                </tr>
            </table>
<?php include ( "./inc/footer.inc.php" ); ?>

So the connection to the database is working. I already checked that. The data is correctly stored in the variables. I manually inserted a 'Test' username via phpMyAdmin in the database and if I put the same username in my website, the code correctly fetches the username from the database and compares it with the input (ergo the result is "Username already exists"). Like I already said, when I fill the form correctly with data, the code terminates but the data is not stored into the database.

Here some version info: Apache/2.4.16 (Unix) PHP/5.5.29

Basically used this guide to set up Apache, PHP, MySQL, phpMyAdmin: Guide for Mac OSX

I checked for errors via die(mysqli_error($link));, and got this response:

Incorrect integer value: '' for column 'id' at row 1 ->Changed argument '' to NULL.

Second error check gave me this:

Incorrect date value: '2015-Nov-Wed' for column 'sign_up_date' at row 1

The NOW() method did the trick. And thanks for pointing out the date mistake: should be $d = date("Y-m-d");