(PHP）setcookie获取错误“；脚本头过早结束”；

我创建了一个PHP脚本，从mysql数据库中读取数据并返回json格式的输出。脚本运行良好，但现在我想将输出保存在cookie中。当我添加函数setcookie时（"Messages"，base64_encode（serialize（$Messages）），157680000000）；我有一个服务器错误500：脚本头过早结束。

如何在cookie中保存数组$messages？

-----------------PHP代码------------------

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); 
header("Last-Modified: " . gmdate( "D, d M Y H:i:s" ) . "GMT"); 
header("Cache-Control: no-cache, no-store, must-revalidate"); 
header("Pragma: no-cache");
header("Content-type: application/json");   
include_once("include/common.php");
if (empty($uname)){
    $uname = (!empty($_GET['u'])) ? $_GET['u'] : "";
    $uname = (!empty($_POST['u'])) ? $_POST['u'] : $uname;
}
if (empty($limit)){
    $limit = (!empty($_GET['limit'])) ? $_GET['limit'] : "";
    $limit = (!empty($_POST['limit'])) ? $_POST['limit'] : $limit;
}
$isonline = is_online();
$user_obj = new User();
$user_info = $user_obj->getUserInfo($uname);
$msg_obj = new Message();
$msg_array = $msg_obj->getMessages($user_info[_USERID_COLUMN],$limit);
$messages = array();
$i = 0;
            [...]
    foreach( $msg_array as $id => $val ){   
            $messages[$i]['id_message']=$val['id_message'];
            $messages[$i]['user']=$val['user'];
            $messages[$i]['user_image']=$val['user_image'];
            $messages[$i]['posted_time']=time2str($val['posted_time']);
            if($options['link'] == "off"){
                $messages[$i]['message']=htmlspecialchars_decode(cleaner($val['message']));
            }else{
                $messages[$i]['message']=htmlspecialchars_decode($val['message']);
            }
            $messages[$i]['mediatype']=$val['mediatype'];
            $messages[$i]['mediaurl']=$val['mediaurl'];
            $i++;
    }   
    array_reverse($messages);
    if(ceckCookie()){
        setcookie("Messages",base64_encode(serialize($messages)),157680000000);
    }
    if( !$isonline && !empty($_COOKIE["Messages"]) ){
        echo($_COOKIE["Messages"]);
    }else{
        echo(json_encode($messages));
    }