PHP sleep function dont work

PHP sleep function dont work

this one :
sleep(30);
echo'test';
die();
work on local (MAMP) but not on my server (Ubuntu, Apache..), I tried to
add set_time_limit(300); It is appair good on phpinfo at
max_execution_time = 300 but still dont work, I still get an error page on
browser (Firefox "The connection was reset").
Any idea ?