Flash 5 email form php browser compatibility

Hi there,
I have just finished putting up a php driven email form on the flash 5 based website http://www.imprentapronto.com. I used the very helpful tutorial on this site (thanks). My problem is that although it works fine for me and a few other people using Mac OSX.3 / Win NT Server / Win XP etc in IE it has problem with older browsers, Netscape 4 shows the site but not the mail forms, IE 5 on Mac 9.2 shows everything fine but won’t send the PHP script. My mission was to create a flash site with the minimum of player problems, so I published as flash 5.
Is this kind of thing normal? is it a PHP problem or a Flash problem?
Any thought would be helpful.