Code to redirect links if domain is changed?

Searching for code that checks if the domain a page is hosted on matches the specified domain. If it doesn't links on the page are redirect somewhere else.

Idea is that when someone steals my CPA landing pages and puts them on their server, the code will redirect to my offers.

I don't think this is possible as whoever that put your page on their server, it become theirs.

Moreover, they are not likely to "steal" your logic wholesale.
i.e.
They can "steal" your graphical presentation (html) of your landing page, but not any php logic coded.

Thus, if they "steal" your html, they would have to change the affiliate link to theirs.
If they are not savy enough to change the affiliate link, then it's still your affiliate link.
But, if they do, it's already out of your control.

Hope that explains.