mod_rewrite is a powerful and sophisticated Apache web server module that allows urls to be rewritten using regular expressions. To redirect the urls to the corresponding url with www in front.
MySQL is a relational database server with a great reputation for being fast, efficient, safe, stable, easy to use, multiplatform (GNU/Linux, * BSD, Windows,...) and well documented. The current MySQL development process is under Oracle's umbrella but the community version is released as free software and you can use under GPL terms.
Apache HTTP is a general purpose web server, designed to achieve a balance between flexibility, portability and performance. Apache HTTP has a modular architecture which allows to extend the basic functionality by selecting a set of modules: the modules are compiled as Dynamic Shared Objects (DSO) and they may be enabled/disabled using the LoadModule directive.
We can use the aptitude command to know if a package has been installed on Debian GNU/Linux or derivative. In CentOS/RHEL/Fedora you can use yum
One of the easiest and safest ways to find duplicate files on GNU / Linux files is using the tool fdupes, fdupes find duplicate files comparing the file sizes, making a check MD5 followed by a byte to byte comparison.
When you install Apache HTTP on CentOS, mod_filter is not enabled by default then if you try to use some of its directives an error like this is raised: "Invalid command 'FilterDeclare', perhaps misspelled or defined by a module not included in the server configuration", so to resolve the above error is enough to add the following line to the httpd.conf.