dotCMS is not normally configured to run PHP on the same server. It can be done, but it is not a standard thing; the file system is also not usually directly accessible (writing a file).
The correct dotCMS implementation here would be to either use the API to create a piece of database content (using the POST from your Arduino), or use a webhook.
I'm not saying you can't use PHP, but I am indicating the reason your question isn't getting any answers - there are a 1000 php-based CMS engines and dotCMS is usually differentiated or chosen because it is Java based.
I actually do a lot of PHP-based integrations with dotCMS, but I use a different server to execute the PHP and then push the contents using one of the tools.
It wouldn't hurt for you to tell us which version you are using, as well, as the rules change quite a bit.
Mark Pitely
Albright College