Password security in PHP scripts

How best do I keep my MySQL database connection password in my PHP script secure from prying eyes?

I have PHP3 scripts running on an Apache web server and connecting to a MySQL database. To connect to a database, my script can specify connection parameters of hostname, username, and password (I say 'can' because defaults are used if these parameters are left blank).

I want access to each database to be protected by a username and password, but I don't want to explicitly hard-code the password in my PHP script. What options do I have of keeping the password hidden from prying eyes but available to my PHP script?

Thanks very much.