How to assign the formatted values ?

Hi,

I have a variable $page_load_time, and with value = 1.7123445
and in template i wrote:
 {assign var="page_load_seconds", 
value=$page_load_time|string_format:"%.4f"}

but {$page_load_seconds} output nothing, what's wrong here?

Anyone can help me ?

Thanks,
james.