Install PHP libevent

Install libevent core library
apt-get install libevent-dev

you will need to install PEAR via apt-get to get the necessary package and distribution system that both PEAR and PECL use.
apt-get install php-pear

Now you will need to install the php5-dev package to get the necessary PHP5 source files to compile additional modules
pecl install libevent

If you get some warning message, try this command
pecl install channel://pecl.php.net/libevent-0.1.0

After install libevent, remember to modify php.ini, add this line
extension=libevent.so

You can use this command to check if php already enable libevent
php -m | grep libevent