PHP(Hypertext Preprocessor) is a server side script language and it is very useful in making dynamic websites by connecting through database(e.g MySQL).It is open source and free.You can install it on Ubuntu 10.04 or any other distribution of linux in many ways e.g by using Synaptic Package Manager or Install it manually after downloading it from it’s website.PHP comes in many packages and modules,so it depends on your work.In general there is a software called LAMP(For Linux),contains all the combination required for web designing.L stands for Linux,A for Apache,M for MySQL and P for PHP.If you have installed other softwares,then correct php modules you can install it by the following method.
Install PHP on Ubuntu 10.04 Lucid Lynx
Using Command Line/Terminal
Just Type the following command at ubuntu terminal(Hit CTRL+ALT+T to open terminal) and enter the login password.
$ sudo apt-get install php5 libapache2-mod-php5
Using Synaptic Package Manager
1.Go to Synaptic Package Manager and search for php.
2.Check the boxes corresponding to the package/module you want to install.
3.Click on Apply to begin the download.It will take few minutes to complete.
4.After completion of download,installation will begin automatically and hardly takes a minutes.(You may have to provide extra details e.g if you want to connect it through MySQL database).