finding out if a key exists in an associative array

I'm creating an associative array that creates keys and values for those keys on the fly. Later I need to check and see if the array contains a key before I call it to avoid the dreaded "Undefined Index" error. Is it possible to see if a key exists in an associative array before I call it? Kind of like the "isset" to check if a variable exists before you call it, and it doesn't error out on you. Thanks, Ron