Fatal error: Call to undefined method PEAR_Error::send()

I'm trying to code a php page that can send a email automatically.
However, i don't understand the error message appeared: Fatal error:
Call to undefined method PEAR_Error::send() in C:\AppServ\www\seeyouu
\send_mail.php on line 22

why is PEAR_Error undefined? i'd already put the PEAR.php and Mail.php
in a same directory but why the error message appear?

appreciate if anyone can tell me.

thank you.