Icinga Web is developed as a separate subproject under the Icinga project umbrella. It is not included in the default Icinga package, hence it needs to be separately installed.
Icinga Web requires a RDBMS backend such as MySQL, PostgreSQL, and Oracle. We will restrict ourselves to MySQL. You can refer to official installation guide for instructions for the other backends. Make sure you have a working MySQL server installation ready for use.
Install the following dependencies required for Icinga Web:
sudo apt-get install php5 php5-cli php-pear php5-xmlrpc php5-xsl php5-pdo php5-soap php5-gd php5-ldap php5-mysql
sudo yum install php php-cli php-pear php-xmlrpc php-xsl php-pdo php-soap php-gd php-ldap ...