过滤数组特定key

//城市
$city = array(
    'beijing' => '北京',
    'shanghai' => '上海',
 'tianjin' => '天津',
);

$result = array_filter(array_keys($city), 'yangyu');

function yangyu($var, $compare = 'shanghai'){
 return $var != $compare ? true : false;
}