Searching for array keys

What's the best way of finding out if a specific array key is in an array? I have an associative array which *may* look like (car =>saab, house => mansion, countyW=> A) but can equally well look like (boat => daycruiser, house => flat, county => B). I want to find out if the key "car" is in the array and do something if it is. I found: $os = array ("Mac", "NT", "Irix", "Linux"); if (in_array ("Irix", $os)){ <do stuff>; } But that only works on the values. But it is exactly what I want to do on keys instead. Suggestions?