ErrorDocument 404 /search.php
<?php
$search_string = basename($_SERVER['REQUEST_URI']);
$filename = $_SERVER['DOCUMENT_ROOT'] . ltrim($_SERVER['REQUEST_URI'], '/');
ob_start();
? >
<html>
<head><title><?php echo "Info about $search_string"; ? ></title></head>
<body>
<?php echo "<h1>$search_string:</h1>\n"; ? >
</body>
<?php
$data = ob_get_contents();
ob_clean();
echo $data."\n(In error handler, saved to $filename)";
$f = fopen($filename, "w");
fwrite($f, $data);
fclose($f);
? >