The imap extension of PHP helps you to work with the IMAP protocol, also with work with NNTP, POP3 and local mailbox access methods.
Install php5-imap
Following command will help you to install on Ubuntu.
sudo apt-get install php5-imap
After installed, enable php5-imap
sudo php5enmod imap
Last thing is RESTART
sudo service apache2 restart
Do you want to verify?
php -m|grep imap
OR
Create a file in web directory name it info.php
vi info.php or nano info.php
Paste below given code in info.php
<?php phpinfo(); ?>
To verify?
Open your site with /info.php
For example: www.yourdomain.com/info.php
