Null Byte Issues

When I change ?p= to a random value, I get an error - of course. When I use a null byte, I see that I am able to negate the “.html” extension the code adds to each file request. If I go to ?p=news.html I get the error Warning: file_get_contents(news.html.html) […]
When I navigate to ?p=news I get the news page, which must be “news.html” So when I add a null byte to the url (?p=news.html%00) I should get the regular news page. Instead I get the error Warning: file_get_contents(news.html) […]. What am I missing? Clearly if I cannot understand this I will not be able to complete this challenge.