Anyone can upload using your PHP form. If you're talking about, without using your PHP form the answer is NO.
What 777 says is that any user on the machine can read, write, execute and delete files in that directory. So the only way a hacker would be able to write to it without your permissions would be if they compromised a linux/unix user account that is already on the system. It's not inherently opened to the internet by having permissions of 777.
