A small form handler

reg.php

<?php
$dir = "/var/www/lena/reg";
$pattern = '/(\d{4}\d{2}\d{2}_\d{2}\d{2}\d{2})/';

if (isset($_GET['list'])) {
  echo "<h1>Liste</h1><ul>";
  foreach (glob("{$dir}/*.txt") as $f) {
    if (preg_match($pattern, $f, $m)) {
      $d = $m[1];
      echo "<li><a href=\"?r={$d}\">{$d}</a></li>\n";
    }
  }
  echo "</ul>";
  exit();
}
else
if ($_GET['r']) {
  $d = $_GET['r'];
  if (preg_match($pattern, $d)) {
    $c = htmlentities(file_get_contents("{$dir}/{$d}.txt"));
    echo "<meta charset=\"utf-8\"><h1>{$d}</h1><pre>{$c}</pre><p><a href=\"?list\">zurück zur Liste</a></p>\n";
  }
  exit();
}

$data = '';
foreach ($_POST as $n => $v) {
  $data .= "$n = $v\n";
}
$fname = strftime("{$dir}/%Y%m%d_%H%M%S.txt");
file_put_contents($fname, $data);

header('Location: https://steinsekte.xyz/reg.html');