Funtion Opendir Error : Warning: opendir(upload/image/activity/$folder_small/) [function.opendir]: failed to open dir: No error in

<?php 
$folder_small = $fetch['folder_small']; * อันนี้ผม Select Database ไว้ด้านบน
$folder_big = $fetch['folder_big']; * อันนี้ผม Select Database ไว้ด้านบน
		
function read_dir($dir, $array = array()){ 
   $dh = opendir($dir); 
    $files = array(); 
    while(($file = readdir($dh)) !== false) { 
       $flag = false; 
       if($file !== '.' && $file !== '..' && !in_array($file, $array)) { 
          $files[] = $file; 
        } 
    } 
    return $files;
}
$path = 'upload/image/activity/$folder_small/';
$folder = read_dir($path);
foreach($folder as $value){
   echo '<a href="upload/image/activity/$folder_big/'.$value.'"><img src="'.$path.$value.'" /></a>&nbsp';
}
?>