I'd say the server uses output buffering. That speeds up the script
execution, but prevents partial output, like you described. The server sends
the page only when the script has completely executed. There is a function
called flush() that you can use to force server to output the buffer, but
from what I know, it doesn't work with IIS6 for example. Don't know about
apache...

here's what the manual says about it:

void flush ( void )

Flushes the output buffers of PHP and whatever backend PHP is using (CGI, a
web server, etc). This effectively tries to push all the output so far to
the user's browser.

flush() has no effect on the buffering scheme of your webserver or the
browser on the client side.

Several servers, especially on Win32, will still buffer the output from your
script until it terminates before transmitting the results to the browser.

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