large files and readfile

I am using php and readfile() to control the download of large files;

These files can be up to 1GB.    There is nothing really exciting or special about the script itself.

The problem I am running into is php is loading the entire file into apache's memory as the file is being read - this seems quite unnecessary. I tired it with fpassthru() as well, with the same result. Why is php loading the entire file into memory? Is there another/better way to download the files (other that resorting to mod_rewrite) that won't use up necessary memory? thanks - daniel