Theres two Possibilities for someone to get your PHP.. 1 of them is not just View Source.. Unless you put the <!-- --> tags in and have PHP in them View Source on the browser will not show it..
Now our two possibilities...
One is already mentioned and thats being stupid and giving someone your FTP information..
The other is not so pleasant and thats if a hacker/cracker
gets into your FTP account and goes and downloads all the PHP files....
Unless your some big corporation or a very well known person whos extremely good At PHP...
I Doubt that someone will goto that extreme..