Not sure if I can use array_intersect or array_search in this case

I have an array ($entry) that could have either of two sets of keys:

"custom_0_first" AND "custom_0_last";

OR

"custom_1_first" AND "custom_1_last";

I'm trying to do the following, but it doesn't seem to be setting the variables:

$firstname = array_search('custom_0_first', $entry) || array_search('custom_1_first', $entry);
$lastname = array_search('custom_0_last', $entry) || array_search('custom_1_last', $entry);

Note that $entry['custom_0_first'] does work fine. I was trying to avoid an IF statement here.

Is my understanding of how array_search or PHP works incorrect? As I understand it, if the first array_search doesn't find the key, the function returns FALSE and then it will check the right side of the OR statement. Is this incorrect? I saw array_intersect that I thought might work, but it looks like that doesn't work with arrays with associative keys.