PHP - Login Page Without Sql That Saves Users And Password On File

<?php
include 'helpfulfunctions.inc';
include 'productsdata.inc';
$user_login_file = 'login.dat';
//var_dump($_POST);
// product data for photo, name, and price.


$alluserinfo = load_users_info($user_login_file);
//validate users info
$errors = array();
if (array_key_exists('register_submit', $_POST)) {

    //check to see if username is taken
    $username_entered = $_POST['username'];
    //check to see if username already exists
    if (array_key_exists($username_entered, $alluserinfo)) {
        $errors['username']['username_exists'] = "Username already exists.";
    }
    //validate username is 4-11 characters long using only a-z A-Z 0-9
    if(preg_match("/^[0-9a-zA-z]{4,11}$/",$_POST['username']) ===0){
        $errors['username']['invalid_username']= "Invalid username. Username must be 4-11 characters long and use only letters and numbers.";
    }
    //validate password "." means any character
    //.* allows numbers 0-9 to be inserted anywhere
    //?= positive lookahead: next text must be like this and follow these rules
    // must be at least 6 characters, contain 0-9, a-z, A-Z
    $pw_entered=$_POST['password'];
    if(preg_match("/^.*(?=.{6,})(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z]).*$/", $_POST["password"]) === 0){
        $errors['password']['invalid_password']="Password must be at least 6 characters and must contain at least one lower case letter, one upper case letter and one digit.";
    }
    //validate that "confirm password" matches password above
    $pw_repeat=$_POST['confirmpassword'];
    if($pw_repeat != $pw_entered){
        $errors['confirmpassword']['pw_no_match']="Passwords do not match. Try again.";
    }
    //validate email format
    $email_entered=($_POST['email']);
    if(!filter_var($email_entered, FILTER_VALIDATE_EMAIL)){
        $errors['email']['invalid_email']="Not a valid email. Please try again.";
    }
//no validation errors=>print invoice
    if (empty($errors)) {
        include 'invoice.inc';
        exit;
    }
}
//reprint if invalid entry. if no errors print invoice
?>

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
    <h3>Please register to continue.</h3>
    <table>
        <tr>
            <td>
    *Username: 
            </td>
            <td>
    <input type="text" name="username"/>                
    <?php
    if (isset($errors['username'])) {
        print implode('<br>', $errors['username']);
    }
    ?>
            </td>
        </tr>
        <tr>
            <td>
    *Password:
            </td>
            <td>
    <input type="password" name="password"/>
    <?php
    if (isset($errors['password'])) {
        print implode('<br>', $errors['password']);
    }
    ?>
            </td>
        </tr>
        <tr>
            <td>
    *Confirm Password: 
            </td>
            <td>
    <input type="password" name="confirmpassword"/>
    <?php
    if (isset($errors['confirmpassword'])) {
        print implode('<br>', $errors['confirmpassword']);
    }
    ?>
            </td>
        </tr>
        <tr>
            <td>
    *Email: 
            </td>
            <td>
    <input type="text" name="email"/>
    <?php
    if (isset($errors['email'])) {
        print implode('<br>', $errors['email']);
    }
    ?>
            </td>
        </tr>
        <tr>
            <td>
    * required info <br>
    <input type="submit" value="Register" name="register_submit">
    <?php
    //print out hiddens with quantities
    save_hidden_qty($_POST['quantity']);
    ?>   
           
    
</form>
 </td>
        </tr>
</table>

<?php

if (!function_exists('load_users_info')) {

 function load_users_info($users_data_file) {
        $fp = fopen($users_data_file, 'r');
//read all lines of login.dat file and create user info arrays
        while (!feof($fp)) {
            $users_info_line = fgets($fp);
            $user_info_parts = explode(',', $users_info_line);
            $user_info_array = array('username' => $user_info_parts[0], 'password' => $user_info_parts[1], 'email' => $user_info_parts[2]);
            $complete_user_info_array[$user_info_array['username']] = $user_info_array;
        }

        fclose($fp);
        return $complete_user_info_array;
    }

}

// function to display products
if (!function_exists('display_products')) {

    function display_products($products_to_display, $quantities = array()) {
        global $errors;
        ?>
        <table border="1">
            <tbody>
                <tr>
                    <td style="text-align: center;"><b><big>Product</big></b></td>
                    <td style="text-align: center;"><b><big>Brand</big></b></td>
                    <td style="text-align: center;"><b><big>Price(each)</big></b></td>
                    <td style="text-align: center;"><b><big>Quantity Desired</big></b></td>
                </tr>
                <?php

                // quantities are 0 unless already inputted, if quantities previously were inputted, return the values. 
                // input boxes
                for ($i = 0; $i < count($products_to_display); $i++) {
                    if (empty($quantities)) {
                        $qty = isset($_POST['quantity'][$i]) ? $_POST['quantity'][$i] : 0;
                        $qty_str = "<input type=text size=3 maxlength=3 name='quantity[$i]' value='$qty'>";
                        if (isset($errors['quantity'][$i])) {
                            $qty_str .= "<span style='font-style:italic;font-size:8px;color:red;'>{$errors['quantity'][$i]}</span>";
                        }
                    } else {
                        $qty_str = $quantities[$i];
                    }
                    // loop to print out table of photo of board, name of the brand, price, and quantity selected
                    printf('
                <tr>
                    <td><img alt="Small" id="lightboxImage"
                             style="width: 119px; height: 88px; bgcolor="#cccccc;"
                             src="http://imgur.com/%s"
                             height="300" width="300"></td>
                    <td style="text-align: center;">%s</td> 
                    <td style="text-align: center;">$%.2f</td>
                    <td style="text-align: center;">' . $qty_str . '</td>
                </tr>
                        ', $products_to_display[$i]['item'], $products_to_display[$i]['board'], $products_to_display[$i]['price']);
                }
                ?>
                <tr><td colspan="4" style="text-align: right; border: none">
                        <input type="submit" value="Purchase"></td></tr>

            </tbody>
        </table>
    <?php

    }

}

if (!function_exists('save_hidden_qty')) {
    function save_hidden_qty($the_quantities){
        foreach ($the_quantities as $key=>$value){
            print "<input type='hidden' name='quantity[$key]' value='$value'>\n";
        }
    }
}

?>