How can I most efficiently check for the existence of a single value in an array of thousands of values in PHP?

A quick way of doing this has been shown below −

if (array_flip($set)[$value] !== null) {
   echo "something"; //take some action

To customize the number of keys, the function can be customized in the below manner −

function array_keys_exists(array $keys, array $arr) {
   return !array_diff_key(array_flip($keys), $arr);