When trying to do a case insensitive comparison between arrays of words, the strcasecmp function works very nicely with this one like so:
$arr1 = array('blue', 'green', 'red');
$arr2 = array('BLUE', 'Purple', 'Red');
$loose_matches = array_uintersect($arr1, $arr2, 'strcasecmp');
print_r($loose_matches) // array('blue', 'red');