What is a PHPinfo file? From the php.net description.
Outputs a large amount of information about the current state of PHP. This includes information about PHP compilation options and extensions, the PHP version, server information and environment (if compiled as a module), the PHP environment, OS version information, paths, master and local values of configuration options, HTTP headers, and the PHP License.
In short, a PHPinfo file is meant to display information about your PHP installation for troubleshooting your ExpressionEngine installation.
To create the file, use a text editor and enter the following.
<?php
// Show all information
phpinfo();
?>
Save this file with a .php extension (you can choose any name) and save it in a public location of your web hosting account (the web root or public_html directory for example.) You can now view the output of this file by directly browsing to the location. In the following example we are assuming the file has been saved with the name of phpinfo.php.
www.example.com/phpinfo.php
You should now be able to see information about your PHP environment which will help the EE support team troubleshoot your installation.
After you are done using this file, you may delete it from your server.
NOTE: This file may act differently on your host as this feature can be disabled. Also, ExpressionEngine also has this feature but this article assumes you have not yet been able to install EE correctly.