Have you check your php.ini ?
I broken my post method once that I set post_max_size the same with upload_max_filesize.
I think that post_max_size must less than upload_max_filesize.
Tested with PHP 5.3.3 in RHEL 6.0
FYI:
$_POST in php 5.3.5 does not work
PHP POST not working
