Yes, PHP is allowed -- in fact, it's the basis of most of the sites running here. There are some functions that are disabled/restricted for security reasons, but if you've tried to use one of those you'll see an error rather than the default page.
I am seeing your "PHP Send Mails" page when I visit your site -- you may want to clear your cache and try again. Do note, though, that sending mail with a spoofed From header is one of the things that you can't do around here, nor can you send mail in any format other than text/plain. That's to prevent the server or domain being blacklisted for spam and phishing. You need to send from an email account linked to your hosting account; anything else will be discarded by an outgoing spam filter.