Implement priority banners.

If the directory "banners_priority" exists and contains files, a file
from it is served 33% of the time, an a file from "banners" 66% of the
time.

This may seem like an odd definition of "priority", but assuming far
fewer priority banners than normal banners, this gives them extra weight
in the selection. Before, any 1 banner in 100 had a 1% chance of being
served; now if that banner is a priority banner it has a 33% chance.

b.php

@@ -1,21 +1,47 @@
 <?php
-//renaming to b.php
-$dir = "banners/";
-$files = scandir($dir);
-$images = array_diff($files, array('.', '..'));
-$name = $images[array_rand($images)];
-
-// snags the extension
-$img_extension = pathinfo($name, PATHINFO_EXTENSION);
-
-// send the right headers
-header('Cache-Control: no-cache, no-store, must-revalidate'); // HTTP 1.1
-header('Pragma: no-cache'); // HTTP 1.0
-header('Expires: 0'); // Proxies
-header("Content-type: image/" . $img_extension);
-header("Content-Disposition: inline; filename=" . $name);
-
-// readfile displays the image, passthru seems to spits stream.
-readfile($dir.$name);
-exit;
+// This script assumes there is at least one normal (non-priority)
+// banner!
+
+// Get the files in a directory, returns null if the directory does
+// not exist.
+function getFilesInDirectory($dir) {
+    if (! is_dir($dir)) {
+        return null;
+    }
+
+    return array_diff(scandir($dir), array('.', '..'));
+}
+
+// Serve a random banner and exit.
+function serveRandomBanner($dir, $files) {
+    $name = $files[array_rand($files)];
+
+    // snags the extension
+    $ext = pathinfo($name, PATHINFO_EXTENSION);
+
+    // send the right headers
+    header('Cache-Control: no-cache, no-store, must-revalidate'); // HTTP 1.1
+    header('Pragma: no-cache'); // HTTP 1.0
+    header('Expires: 0'); // Proxies
+    header("Content-type: image/" . ext);
+    header("Content-Disposition: inline; filename=" . $name);
+
+    // readfile displays the image, passthru seems to spits stream.
+    readfile($dir.$name);
+    exit;
+}
+
+// Get all the banners
+$bannerDir = "banners/";
+$priorityDir = "banners_priority/";
+
+$banners = getFilesInDirectory($bannerDir);
+$priority = getFilesInDirectory($priorityDir);
+
+// If there are priority banners, serve 1/3rd of the time.
+if($priority !== null && count($priority) !== 0 && rand(0,2) === 0) {
+    serveRandomBanner($priorityDir, $priority);
+}
+
+serveRandomBanner($bannerDir, $banners);
 ?>