I found out a solution for Apache 2. However this works only without threads and only on POSIX compatible OS systems (e.g. Linux, OpenSolaris...).
<?php
// Terminate Apache 2 child process after request has been
// done by sending a SIGWINCH POSIX signal (28).
function kill_on_exit() {
posix_kill( getmypid(), 28 );
}
register_shutdown_function( 'kill_on_exit' );
?>