[CentOS] Problem installing WordPress on CentOS

<snip>

> And here's where things go wrong. When I click on "Begin installation",
> Firefox wants to "open" the file install.php by leaving me one of the
> two following choices :
>
> a) edit "install.php" in GEdit
> b) download "install.php"

It sounds like you might not have your php set up correctly in apache or 
at least in this virtualhost.  Somewhere in your configuration do you have 
lines that say:

AddHandler php5-script .php
AddType text/html .php

This is most likely found in /etc/httpd/conf.d/php.conf

Barry