Problem with big files

we would like to upload large files to the server ( 20 MB ). We have changed the configuration of php.ini in order to upload this files via http. We changed the following parameters: post_max_size = 25M file_uploads = On upload_tmp_dir =/tmpupload upload_max_filesize = 25M memory_limit = 25M also we set in the script that makes the upload the function set_time_limit(0), so the script has no time out. The problem appears when the file is larger than 6.5 MB. It shows an error telling us that the page could not be found. Thanks