Display page by category

Hello,

I tried a lot to do this but I didn’t so maybe you can help me.
I want to modify the single.php page so that at the end of each article it will display a different php page depending on the category of the article.
I don’t know what to write inside the if condition to display php pages by categories inside single:

if(????){
include(aaa);
}
else
include(bbb);

Thanks