更新HTML与javascript在php的目录大小的间隔

我试图获得每3秒计算一次的目录大小。计算成功，大小显示出来，但没有更新。下面是当前的代码:

<script type='text/javascript'>
    window.onload = sizeRefresh;
    function updatesize() {  
        $dir_size = <?php 
                        $dir = "pics"; 
                        function dirSize($directory) { 
                        $size = 0; 
                        foreach(new RecursiveIteratorIterator(new RecursiveDirectoryIterator($directory)) as $file){ 
                        $size+=$file->getSize(); 
                        } 
                        echo json_encode(round($size/1024,0) . " KB<br>" . round($size/1024/1024,0) . "MB<br>" . round($size/1024/1024/1024,0) . "GB" ); 
                        } 
                        dirSize($dir) 
                        ?>;
        var hsize = document.getElementById('sizemark');
        hsize.innerHTML = $dir_size
        }

    function sizeRefresh() {
        setInterval("updatesize()", 3000);
    }
</script>
<h1 id='sizemark' > size </h1>

我做错了什么?

谢谢。