I was able to generate (I presume, though I didn't actually download the file offered) so that may work just fine, but your preview funcitonality is definitely borked as MarkR mentioned. I received a different error though:
Warning: file_put_contents(e6fdd60c9e6ff2b4a239114faae22454/) [function.file-put-contents]: failed to open stream: Is a directory in /home/detad/public_html/freephpgenerator.com/generator.php on line 79
Anyway, it's a neat idea but it needs some cleanup. Also, have you made absolutely certain that you have taken precautions to prevent people traversing your file tree and/or revealing your own underlying code and database information? Not to mention the possibility of running a website from your domain, spam mailing, etc., etc., etc.
Running user-entered PHP on your site is extraordinarily dangerous so I hope you have been VERY careful with filtering what code actually gets executed on your server.