<?php
if(isset($_POST['field1']) && isset($_POST['field2'])) {
    $data = $_POST['field1'] . '-' . $_POST['field2'] . "<br>";
    $ret = file_put_contents('user.txt', $data, FILE_APPEND | LOCK_EX);
    if($ret === false) {
        die('There was an error writing this file');
    }
}
?>