Submit email form to .txt file won't work

I have made a script with a form which is supposed to submit a persons email to a .txt file, only problem is that nothing happends to the .txt file, it is kept blank when the function is called. Both the html file and the php file is kept in the same folder and the .txt file is named formdata.txt .

Html code:

<form name="newsletter-form" action="process-form-data.php" method="post" id="newsletter-form">
            <input type="email" name="newsletter-email" id="newsletter-email" class="form-control" placeholder="Enter Your Email" data-validate="validate(required, email)" />
            <input type="submit" id="newsletter-submit" class="btn" value="Notify Me" />
        </form>

Php code named process-form-data.php:

    <?php

// Receive form Post data and Saving it in variables
$email = $_POST['newsletter-email'];

// Write the name of text file where data will be store
$filename = "formdata.txt";

// Marge all the variables with text in a single variable. 
$f_data= '
Email :  '.$email.'
=========================
';

echo 'Form data has been saved to '.$filename.'  <br>
<a href="'.$filename.'">Click here to read </a> ';
$file = fopen($filename, "a");
fwrite($file,$f_data);
fclose($file);
?>