php, change a db: from MySQL to txt file
require "includes/connect.php";

$msg = '';

if($_POST['email']){

    // Requested with AJAX:
    $ajax = ($_SERVER['HTTP_X_REQUESTED_WITH']  == 'XMLHttpRequest');

    try{
        if(!filter_input(INPUT_POST,'email',FILTER_VALIDATE_EMAIL)){
            throw new Exception('Invalid Email!');
        }

        $mysqli->query("INSERT INTO coming_soon_emails
                        SET email='".$mysqli->real_escape_string($_POST['email'])."'");

        if($mysqli->affected_rows != 1){
            throw new Exception('This email already exists in the database.');
        }

        if($ajax){
            die('{"status":1}');
        }

        $msg = "Thank you!";

    }
    catch (Exception $e){

        if($ajax){
            die(json_encode(array('error'=>$e->getMessage())));
        }

        $msg = $e->getMessage();
    }
}

<?php

error_reporting(E_ALL ^ E_NOTICE);

$db_host = '';
$db_user = '';
$db_pass = '';
$db_name = '';

@$mysqli = new mysqli($db_host, $db_user, $db_pass, $db_name);

if (mysqli_connect_errno()) {
die('<h1>Could not connect to the database</h1><h2>Please try again after a few moments.</h2>');
}

$mysqli->set_charset("utf8");


?>

$file = fopen('path/to/file', 'r+');

    fputs($file, 'email@domain.com');
    // fputs() is an alias for fwrite, they are interchangable