Verifying site ownership [duplicate]

This question already has an answer here:

• programmatically determining if someone owns a website? 7 answers

First off, apologies about the silly question, but I could not find documentation on how to solve this problem.

I have a SaaS project, in which users have to register in order to user this service. This project is written in PHP.

Now, at one point during the registration, users have to submit their website url (because this SaaS solution is specific to the domain being registered by the user).

My question is after a user submits his/her website during registration, how do I verify that he/she is the owner of the site they say they own?

In case you are wondering, I have to verify this in order to prevent errors and prevent fraud.

Thanks.