Note that checking the existence of a subkey of an array when that subkey does not exist but the parent does and is a string will return false for empty.
i.e.
<?php
$params = array('search'=>'1');
empty($params['search']['filter']); # returns false
?>
This is correct, per the documentation (http://php.net/manual/en/language.types.string.php); quite a bit down the page is the Warning: "Writing to an out of range offset pads the string with spaces. Non-integer types are converted to integer." ) I didn't receive a warning but perhaps that's correct too...depends on whether the string -> integer conversion is considered "illegal": "Illegal offset type emits E_NOTICE."
(i.e. since $params['search'] is a string, the 'filter' subscript is converted to 0, so the test becomes empty($params['search'][0]), which is obviously false), but it tripped me up enough to mistakenly file a bug report (which I've since closed).