Trouble figuring out a Walk through an Array

Greetings All, Having trouble figureing out a certain walk through an array. And I am not finding the help I need in the manual or anything. I have an associative arr ($arr) With about 20 elements in it. Ten of which are required. So I have another associative array called $required, with all the elements in the array that are required. I have initaited a foreach loop on the $arr splitting things into $key and $value. Now the part I am having trouble with is this. I want to see if $key matches any of the values in $required. If so do something in particular. If not then do something else. It does not particularly matter which value in the $required it matches, just so long as it matches. Thanks for any help. Phillip