<?php
echo "Hello World!";
echo "Date and Time now is :". date('d-m-Y, g:i a');
?>

Hello World!
Date and Time now is :24-07-2008, 1:44 am

PHP files are usually stored with a .php extension in your webserver.
<?php tag specifies the start of your PHP code and the ?> specifies the end of PHP code. 'echo' in PHP, is used to print something on the screen.
The date() function in PHP takes a number of parameters and returns the date in the desired format. For more info on the parameters that the date() function can take, visit here.
Download PHP and also find Installation instructions here.