Port the code to PHP 8.5.

Replace the short open tags and the removed functions (mcrypt, mysql_error,
salt-less crypt). Decode the reCAPTCHA reply as JSON, turn off mysqli
exceptions, drop stripslashes(), and default missing input values.
Fix the misplaced continue in prune_images().

display.php

@@ -1,4 +1,4 @@
-<?
+<?php
 require_once('./open_db.lib.php');
 open_db('open');
 require_once('./get_data.lib.php');
@@ -37,7 +37,7 @@ function display_img($id) {
   }
 
   $query = "SELECT img, mime FROM $table WHERE 1 AND ID = ".$db->real_escape_string($id)." LIMIT 0, 1";
-  $result = $db->query($query)or die("Query failed : ".mysql_error());
+  $result = $db->query($query)or die("Query failed : ".$db->error);
 
   $row = $result->fetch_assoc();
   if ($row) {