How to search a word in a lot of files in PHP

How to search a word in a lot of files in PHP

I am having a problem with my PHP script. I am doing a simple PHP search
engine, and I won't use MySQL, no way!! I am trying to find a word in some
PHP files, in a subdirectory of a directory. I have the page that will
have the engine and in the same directory I have a folder called Files,
and in the folder Files, I have a lot of subdirectories, like 'PHP' or
'HTML'. I wan't to search the word that was written in the text box in all
the files of the folder 'PHP', that is on the Files directory. I am almost
sure that I'll need to use a array... And I want to show the results on
the page. Anybody knows how?