sorting images in javascript

<!DOCTYPE html>
<html>
<head>
<title>http://stackoverflow.com/questions/16881351/sorting-by-upload-modified-date-php-code</title>
<style type="text/css">
        #container img { width:100px; height:100px; margin-right:1em; display:inline-block; }
</style>
<script type="text/javascript">

var container, images, arr;

function prepare_images(){
        container = document.getElementById('container');
        images = container.getElementsByTagName('img');//nodeList object
        arr = [];//array object
        for (var i=0;i<images.length;i++) {
                arr[i] = images[i];
                arr[i].onmouseenter = function(){
                        var str = this.getAttribute('data-m') + ' : ' + this.getAttribute('data-u') + ' : ' + this.getAttribute('alt');
                        document.getElementById('test').value = str;
                };
        }
        sort_images('data-m');
}

function sort_images(attr,isNum){
        arr.sort(function(a,b){
                return isNum? parseInt(a.getAttribute(attr),10) - parseInt(b.getAttribute(attr),10) :
                        a.getAttribute(attr).toLowerCase().localeCompare(b.getAttribute(attr).toLowerCase());
        });
        for (var i=0;i<arr.length;i++) {
          container.appendChild(arr[i]);
        }
}

if (window.addEventListener) {
        window.addEventListener("load", prepare_images, false); // for all browsers
} else if (window.attachEvent) {
        window.attachEvent("onload", prepare_images); //for IE5-8
}

</script>
</head>
<body>
<button onclick="sort_images('data-m',true);">sort images by modification time</button>
<button onclick="sort_images('data-u',true);">sort images by upload time</button>
<button onclick="sort_images('alt',false);">sort images by alt name</button> <br/>
<input id="test" type="text" value="" style="width:100%" />
<div id="container">
<?php
//phpinfo();
$dir = 'files';
$file_display = array('jpg', 'jpeg', 'png', 'gif');
if(file_exists($dir) == false){
        echo 'Directory \''.$dir,'\' not found!';
} else {
        $dir_contents = scandir($dir);
        //usort($dir_contents, create_function('$a,$b', 'return filemtime("'.$dir.'/$a")<filemtime("'.$dir.'/$b");'));
        foreach($dir_contents as $file){
                if($file == '.' || $file == '..') { continue; }
                $arr = explode('.',$file);
                $file_type = strtolower(end($arr));
                if(in_array($file_type, $file_display)){
                        echo '<img src="', $dir, '/', $file,'" alt="', $file,
                                '" data-m="', filemtime("$dir/$file"),
                                '" data-u="', filectime("$dir/$file"), '" />';
                }
        }
}
?>
</div>
</body>
</html>