PEAR – PHP Extension and Application Repository, PEAR is a framework and distribution system for reusable PHP components.
The mail() function is using for sending emails from a php script. This is the most simplest way to send emails from a server and also its not secure process.

The mail sending scripts will shows some errors/warning while sending emails due to the absence of mail function[mail()].

Solution:
Step 1: Check PhPear is already installed or not

# rpm -qa|grep php-pear
#

If it is not installed on the server, please install it by executing the foillowing command from the command line;

# yum install php-pear
# rpm -qa|grep php-pear
php-pear-1.4.9-8.el5

Step 2: Install Php-mail extension by executing the following command;

# pear install Mail

Step 3: Restart webserver

# service httpd restart

Check