PHP: exec() doesn't run in the background even with ">/dev/null 2>&1 &"

I'm calling this in my php script:

    exec("gutschein.php >/dev/null 2>&1 &");

Calling the script (generates a pdf and sends it away by e-mail) works, but the process is not running in the background (I checked it out with a sleep statement inside gutschein.php). The browser is hanging until execution of gutschein.php is finished.

I also checked out the following:

    exec("/usr/bin/php gutschein.php >/dev/null 2>&1 &");

or

shell_exec("/usr/bin/php gutschein.php >/dev/null 2>&1 &");

It doesn't change anything. The script is actually running on a linux server. Has anybody an idea what I'm doing wrong?