This has been the case for a while, and it's not just PHP. You'll notice if you try to quote theymos' post from 2014 explaining how the Legendary activity requirement is calculated, you'll meet the same error, triggered by the "concat" function. Remove/rename this function and it will let you post the code quite happily.

Try it yourself. You will be hit with a CloudFlare "Sorry, you have been blocked" page. You don't even need to try to make the post; hitting "preview" will do the same thing.