To install this on your server you must force your webserver to execute /robots.txt as PHP.
In apache there are two ways to make your robots.txt execute PHP
NOT RECOMMENDED: Make ALL .txt files execute PHP by adding the following line to .htaccess or server config
AddType application/x-httpd-php .txt
A safer method: use mod rewrite, something like:
RewriteRule ^/robots.txt(.*?) something.php?uri=$1 [P,L]