php如何返回json

header('Content-Type:application/json; charset=utf-8');
$arr = array('a'=>1,'b'=>2);
exit(json_encode($data));

header('Content-Type:text/xml; charset=utf-8');
exit($xml);

$arr = array('a'=>1, 'b'=>2, 'c'=>3);
$json = json_encode($arr);
$callback = $_GET['callback'];
exit($callback."($json)");

$this->ajaxReturn (json_encode($arr),'JSON');
$this->ajaxReturn (json_encode($arr),'JSONP');
$this->ajaxReturn (json_encode($arr),'XML');

json_encode('中文',JSON_UNESCAPED_UNICODE);

header('Content-Type:application/json; charset=gbk');

$data = $db->select($sql);
$data = json_encode($data);
$data=preg_replace("#\\\u([0-9a-f]{4})#ie", "iconv('UCS-2BE', 'gbk', pack('H4', '\\1'))", $data);

exit($data);

$sjson = json_decode($res);  
for($i=0;$i<count($sjson);$i++)
{ 
    echo $sjson[$i];
}