As I understand it, $PHP_SELF is the same thing as $SCRIPT_NAME, both of which deliver the script name and the path of the script: (ie. "/development/mydir/testingcode.php").
I guess Netscape just doesn't like that. Try changing your code to read:
<FORM ACTION="<?php echo $SERVER_NAME . $SCRIPT_NAME; ?>" METHOD="POST">
which will display myserver.com/development/mydir/testingcode.php