No, I'm not.
It turned out that it was the Apache configuration on a virtual Linux webserver.
PHP is a part of Apache on that and it won't let me make temporary files, that are needed when you want to upload some files.
I've tried the same script on a different server and it worked just as I wanted it to do.
On the first server I got this error-message:
Warning: SAFE MODE Restriction in effect. The script whose uid is 1044 is not allowed to access /tmp/php9sX4TC owned by uid 0 in /opt/guide/www.domain.nl/HTML/test/submit.php3 on line 13
Someone got a clue what to do so I can still upload a file on the first server?
Patrick