either way is just as secure
if you put it in the php source code, it's not displayed to the user, (unless there is an error and it displays source code).
if you have the admin account in a DB you still have to have the db password in the php, so the password is still just as exposed to any 'hacker' who would have got the password by reading your source code anyway, because by the time they've seen the source, they seen all the code and know the DB passwords and how to get the passwords from the DB
storing the password in the DB is a lot more convenient.