if you are getting this error on your php script, you probably are trying to run a script by sending email through smpt right. i was trying to do the same thing and i was getting this error too,

basically what you have to do is install the mail pacakage in php pear in order to use this
but how do you install it? installing it is easy

this is how i did it. i logged in as root and send this command:

yum install php-pear-Mail

after the installation was completed, i was able to see the Mail.php in /usr/share/pear/