codice:
$found = '';
$id = 3978;
$len = strlen($id.'_');
foreach($dir as $v)
	if(strpos($v, $id.'_') === 0)
		$found .= ''.substr($v, $len).'
';
echo $found;


codice:
$found = '';
$id = 3978;
$len = strlen($id.'_');
foreach($dir as $v)
	if(substr($v, 0, $len) === $id.'_')
		$found .= ''.substr($v, $len).'
';
echo $found;



[edit]
codice:
$id = 3978;
$filter_prefix = create_function('&$v', 'return (strpos($v, "'.$id.'_") === 0);');
$remove_prefix = create_function('&$v', 'return "".substr($v, strlen("'.$id.'_"))."";');
echo implode('
', array_map($remove_prefix, array_filter($dir, $filter_prefix)));