safe mode, upload_tmp_dir - case with upload

hello PHP Masters:)
my privider set:
safe_mode to ON and leave upload_tmp_dir blank
I think that there is the reason for my trouble with upload files via
POST method. If yes - I will ask my provider for changes but first I
would like to ask You. Can you ensure me?
thx in advance,
Rob

PS If I am wrong, the explanation: files are uploaded to tmp dir, but
if I check by file_exists() the result is false. If I use
move_uploaded_file() the result is the same: false.