<?php
$ENCO = "";
$DECO = "";
if ($_POST['decoded'] != "") {
  $ENCO = '<'.'?'.'php
$N="'.base64_encode(stripslashes($_POST['decoded'])).'";eval(base64_decode($N));
?>';
}
if ($_POST['encoded'] != "") {
  
  if (strpos($_POST['encoded'], "\"") > 0) {
    $Start = strpos($_POST['encoded'], "\"");
    $End = strpos($_POST['encoded'], "\"", $Start+1);
  }
  else {
    $Start = strpos($_POST['encoded'], "'");
    $End = strpos($_POST['encoded'], "'", $Start+1);
  }
  $DECO = base64_decode(substr($_POST['encoded'], $Start, $End-$Start));
}
?>
<form action="test.php" method="POST">
dekodiert:<br>
<textarea name="decoded" rows="14" cols="60"><?= $DECO ?></textarea><br>
<input type="submit" value="encode">
</form>
<form action="test.php" method="POST">
Enkodiert:<br>
<textarea name="encoded" rows="14" cols="60"><?= $ENCO ?></textarea><br>
<input type="submit" value="decode">
</form>