在wordpress中创建rest api时无法修改头错误

I am trying to create a REST API which generate json data for wordpress posts. The code below is in functions.php file:

add_action('wp_enqueue_scripts', 'create_json_data', 0);
function create_json_data(){
    ob_start();
    if (isset($_GET['getjson'])  && $_GET['getjson'] == true) {
        if (have_posts()) {
            $data = array();
            while (have_posts()) {
                the_post();
                $new_data = array(
                    "title" => get_the_title(), 
                    "content" => get_the_excerpt(), 
                    "image" => get_the_post_thumbnail_url(),
                    "hyperlink" => get_the_permalink()
                    );
                array_push($data, $new_data);
            }
        }
        header('Content-type: application/json');
        echo json_encode($data);
        exit();
    }
    ob_end_flush();
}

It runs very well in newly wordpress installation. But when I implement it in my online website it gives me error

Warning: Cannot modify header information - headers already sent by (output started at /home1/public_html/blogsite/wp-content/themes/webtheme/header.php:10) in /home1/public_html/website-theme/wp-content/themes/webtheme/functions.php on line 159

at line 10 in header.php is wp_head()

I don't know how to get ride of this error to generate json data only.

You need to create custom AJAX action. You will find everything you need on the codex : https://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_(action)

Or you can use the Wordpress REST API plugin. It's allow you to create your own API endpoint. https://wordpress.org/plugins/rest-api/

EDIT I give you a little example, with the jQuery ajax code to call it.

add_action('wp_ajax_XXXXXX', 'ajax_XXXXXX');
add_action('wp_ajax_nopriv_XXXXXX', 'ajax_XXXXXX');
function ajax_XXXXXX() {
    header('Content-Type: application/json');
    echo json_encode(array(
        'text' => "Lorem ipsum dolor ...",
        'time' => time(),
        'user_id' => get_current_user_id()
    ));
    die();
}

$.ajax({
    url : "/wp-admin/admin-ajax.php",
    method : "POST",
    data : {
        action : "XXXXXX"
    },
    success : function(datas) {
        console.log(datas);
    }
});