What is the difference in the way php curl and browsers access scripts?

what is the difference between the way php curl and browsers access scripts?
now requests to an api can correctly return a value by accessing the script in a browser, while sending a post with php-curl returns an error saying that the value of content-length has not been set. What"s the difference between the two? What does the nginx agent do during the process?

$topic = "topic";
    $projectId = "projectid";
    $title = "hahaha";
    $content = "lol";

    $payload = array(
        "message" => array(
            "topic" => $topic,
            "notification" => array(
                "title" => $title,
                "body" => $content,
            )
        )
    );

    $json = json_encode(trim($payload));
    $headers = array(
        "Authorization:Bearer ".$this->getFcmApiAccessToken(),
        "Content-Type: application/json; UTF-8",
        "Content-Length:".strlen($json),//"Content-length:0"
    );

    $ch = curl_init();
    curl_setopt($ch,CURLOPT_URL,"https://fcm.googleapis.com/v1/projects/{$projectId}/messages:send");
    curl_setopt($ch,CURLOPT_POST, true );
    curl_setopt($ch,CURLOPT_HEADER, 1);
    curl_setopt($ch,CURLOPT_HTTPHEADER,$headers);
    curl_setopt($ch,CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch,CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch,CURLOPT_SSL_VERIFYHOST, false);
    curl_setopt($ch,CURLOPT_POSTFIELDS, $json);
    curl_setopt($ch,CURLOPT_RETURNTRANSFER, 1);

    $result = curl_exec($ch);
    curl_close($ch);

    var_dump($result);exit;