Wordpress自定义帖子模板

I'm trying to get Wordpress to include a different template file, within single.php, if it sees a file matching the slug.

The file does exits, path is correct, safe_mode set to OFF...am I missing something?

 $dir=get_bloginfo('stylesheet_directory').'/post_tmpl/';
 $categories=get_categories();

 foreach($categories as $cat){
    if(is_file($dir.$cat->slug.".php")){
        require($dir.$cat->slug.".php");
    }else{
        require($dir."default.php");
    }
 }

can you try with this code?

$dir=get_bloginfo('stylesheet_directory').'/post_tmpl/';
 $categories=get_categories();

 foreach($categories as $cat){
    $temp = $dir.$cat->slug;

    if(is_file($temp.".php")){
        require($temp.".php");
    }else{
        require($dir."default.php");
    }
 }

Thanks