write.php

<?php
    header("Location: " . $_SERVER['HTTP_REFERER']);

    $post = $_POST['content'];

    $post = htmlspecialchars($post);
    $post = filter_var($post, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_HIGH);
    $post = str_replace("\n", " ", $post);
    $post = str_replace("\r", " ", $post);

    // Test if post matches last post
    $first_line = fgets(fopen("content.txt", 'r'));
    $format_post = "- " . $post . "\n";
    if ($first_line == $format_post) {
        exit();
    }

    if ($post != "" && strlen($post) <= 400 && ! ctype_space($post)) {
        file_put_contents("content.txt",  "- " . $post . "\n" . file_get_contents("content.txt"));
    }
?>