php: checking an array value for non-zero, non-null

For a long time, I used:
if($array['key']) { Do stuff; } // This used to check if the value was set and non-zero, non-false, non-null
but a while back, that stopped working on some servers if $array['key'] wasn't set and I started using:
if(isset($array['key']) && $array['key']) { Do stuff; }

I never really dug into it but I am getting tired of having to be so verbose. Is there a direct way to do what I was doing before or do I just have to check for both conditions separately? What changed?