Smarty foreach循环和file_exist

I'm using the Smarty php template engine, and I'm trying to verify the existence of .jpg images with an $id_attribute name:

{foreach from=$product.colors key='id_attribute' item='color'}
    {if file_exists($img_col_dir|cat:$id_attribute|cat:'.jpg')} 
        yes 
    {else} 
        no 
    {/if}

The $img_col_dir is "http://www.domain.com/img/co/" and $id_attribute is a number. Many files exists in .jpg format, but the whole loop doesn't find anything, returning "no". What am I doing wrong?

You can't check for existance of a file on a url,

http://www.domain.com/img/co/myfile.jpg

only on your local drive

c:\myfolder\myimages\myfile.jpg

also, you should do this logic in your php file, not in your smarty template.

Check if the file exists then pass through a smarty variable image_exists for each image.