What PHP is asking for here is a mail server that allows you to connect and send mail through.
Yahoo does NOT offer free POP/SMTP access (Gmail does! :p).
What about using your ISP's email server? Simply fill in the info for that instead.
NOTE: I'm not sure what Gmail's policy is about using a PHP script to send e-mail via their SMTP server. I know they allow free POP/SMTP access to their users but.. I haven't read their complete policy. If you would like a free Gmail account, PM me with the e-mail address I can contact you at.
EDIT: After I posted, I thought about it; Gmail uses SSL on their SMTP. You might run into problems with that, not sure; never used it. I use a PHP mailer class called "PHPMailer" by Brent R. Matzelle.