Based on https://stackoverflow.com/a/1461363/1641835:
$someArray = array('fb' => 32, 'gp' => 11, 'tw' => 7, 'vk' => 89, 'ok' => 112);
$max_keys = array_keys($someArray, max($someArray));
// $max_keys would now be an array: [ 'ok' ]
$max_keys
will now be an array of all the keys that point to the maximum value. If you know there will only be one, or you don't care which you retrieve, you could instead use:
$someArray = array('fb' => 32, 'gp' => 11, 'tw' => 7, 'vk' => 89, 'ok' => 112);
$max_key = array_search(max($someArray), $someArray);
// $max_key would now be 'ok'