What is a phpinfo() file
From Online Manual
A phpinfo.php file is a file that makes it easier for us to see information about your server. While it doesn't include version information about your database scheme, it does include version information about PHP, Apache/IIS, GD, and other things. It also contains a description of many important settings, which could possibly be causing problems with your installation.
Creating a phpinfo.php file is simple and easy. With a simple text editor (such as Notepad), create a new "phpinfo.php" file containing this code, and only this:
<?php phpinfo(); ?>
Upload this page to your server, and call it with your Internet browser. Now a page should be displayed telling you the PHP version installed on your server and many more detail information. When you ask a question to the team, give us also a link to this phpinfo.php file.
If an empty page or some cryptic code such as
<?php phpinfo(); ?> is displayed, your server does not run PHP and you cannot install a Simple Machines Forum on your server. You will have to either install and configure PHP on your server (if you are the owner of your server) or move to a new host (if you only are a client) in order to run SMF.