What do you do with html injections?

[eluser]Mr.Data[/eluser]
Hello!
The page I created contains almost everywhere some forms where the user can type in some content which is displayed afterwards. If someone types in some html code I don't want this code to be rendered but it should be cleaned by a php function like htmlentities(). It is possible that I manually add the htmlentities() function to each output but can't you do this globally? What do you do if you come into such a situation?