cUrl don’t sync when cookie in header is big

    $cookies_str = '';
    for ($i=0; $i < 1000; $i++) { 
        $cookies_str .= "test{$i}=testtestestestsetse{$i}; ";
    }
    $param = [
        //set params here
    ];
    $param = http_build_query($param);
    ///
    $url = "http://localhost/get_test.php";
    $cUrl = curl_init($url);
    curl_setopt($cUrl, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($cUrl, CURLOPT_POSTFIELDS, $param);
    curl_setopt($cUrl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($cUrl, CURLOPT_TIMEOUT, 5);
    curl_setopt($cUrl, CURLOPT_CONNECTTIMEOUT, 5);
    curl_setopt($cUrl, CURLOPT_HTTPHEADER, [
        'Content-Type: application/x-www-form-urlencoded; charset=UTF-8',
        'Content-Length: ' . strlen($param)
    ]);
    curl_setopt($cUrl, CURLOPT_COOKIE, $cookies_str);
    //execute post
    $data = curl_exec($cUrl);
    //close connection
    curl_close($cUrl);

When I set $i=10, curl_exec will do sync. But when I set $i=1000, curl_exec will do async (not sync).

How to force curl_exec always do sync ?

Please help ! Thank you for advanced.