Here’s a very handy script which will scan through a directory of images, form an array of the contents of that directory and then return the array in JSON format to the client-side where the images will be preloaded into the user’s cache.

There are two components to this: the PHP script and some simple JavaScript:

The Code (PHP):

/* PHP file, e.g. scanImageDirectory.json.php */
 
// Check that a callback function has been specified:
if (!isset($_GET['callback']) || !isset($_GET['directory'])) exit;
 
// Use PHP5's scandir function to scan all
// of images directory:
$dirContents = scandir($_GET['directory']);
 
// Define function to confirm each
// filename is a valid image name/extension:
function isImageFile($src) {
    return preg_match('/^.+\.(gif|png|jpe?g|bmp|tif)$/i', $src);
}
 
// Loop through directory files and add to
// $arrayContents on each iteration:
$arrayContents = '';
foreach($dirContents as $image) {
    if (isImageFile($image)) {
        $arrayContents .= !empty($arrayContents) ? ',' : '';
        $arrayContents .= '"' . 'images/' . $image . '"';
    }
}
 
// Prepate JSON(P) output
$output = $_GET['callback'] . '({\'images\':[' . $arrayContents . ']});';
 
// Output the output:
echo $output;

The above PHP script will use PHP5′s scandir to scan the ‘images’ directory and it outputs a list of all images in JSON format. (so it can be used by our JavaScript). The JavaScript is below:

The Code (JavaScript):

/* JavaScript Component */
 
function preloadImagesFromDirectory(dir) {
 
    if(!dir) return;
 
    function getJSON(URL,success){
 
        // Create new function (within global namespace)
        // (With unique name):
        var uniqueID = 'json'+(+(new Date()));
            window[uniqueID] = function(data){
                success && success(data);
            };
 
        // Append new SCRIPT element to DOM:
        document.getElementsByTagName('body')[0].appendChild((function(){
            var script = document.createElement('script');
            script.type = 'text/javascript';
            script.src = URL.replace('callback=?','callback=' + uniqueID);
            return script;
        })());
 
    }
 
    function preload(srcArray) {
        for(var i = 0; i < srcArray.length; i++) {
            (new Image()).src = srcArray[i];
        }
    }
 
    // Get that JSON data:
    getJSON('scanImageDirectory.php?directory=' + encodeURIComponent(dir) + '&callback=?', function(data){
        return data.images ? preload( data.images ) : false;
    });
 
}

The above JavaScript uses JSONP to load the PHP file into a SCRIPT element as if it were a JavaScript file itself, then the automatically generated function is fired which then executes the specified callback (‘success’) function which is specified when calling getJSON. The callback then preloads the passed data (i.e. the array of images).

Usage:

// We don't want to disturb anything so we'll wait
// until everything's done loading before preloading:
window.onload = function(){
    preloadImagesFromDirectory('images/');
}

Security concerns:

Since the directory to be scanned is specified in a query string it would be VERY easy for ANYONE to gain information on your file structure/nomenclature. The regular expression used to validate the image names on the server-side (the PHP script) will stop any files from being listed that do not end in gif, png, jpg, jpeg, bmp or tif. If you’re still concerned then simply specify the directory to be scanned from within the PHP script instead of having it passed through the query string.