利用官方接口实现内容的同步的几个步骤：
1.获取token接口，appid和secret可以从微信公众号后台查看。

https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=你的appid&secret=你的secret";

2.上传封面图片，获取返回的id

https://api.weixin.qq.com/cgi-bin/material/add_material?access_token=$token

3.构造上传的内容
4.发送构造的内容

https://api.weixin.qq.com/cgi-bin/material/add_news?access_token=$token

完整代码，在functions.php中添加即可。

//获取微信公众号的token
function GetWeixinToken(){
    //$headers = array("Content-type: application/json;charset=UTF-8","Accept: application/json","Cache-Control: no-cache", "Pragma: no-cache");
    $url="https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=你的appid&secret=你的secret";
    $curl=curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
    //curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    $output =curl_exec($curl);
    //var_dump($output);
    $result=json_decode($output);
    curl_close($curl);
    //var_dump($result->access_token);
    // var_dump($url);
    return $result->access_token;  
}

//构造文章信息
// {
//     "articles": [{
//      "title": TITLE,
//     "thumb_media_id": THUMB_MEDIA_ID,
//     "author": AUTHOR,
//     "digest": DIGEST,
//     "show_cover_pic": SHOW_COVER_PIC(0 / 1),
//     "content": CONTENT,
//     "content_source_url": CONTENT_SOURCE_URL,
//     "need_open_comment":1,
//     "only_fans_can_comment":1
//     }]
// }
function GetArticalInfo($token){
    global $post;
    $postid=$post->ID;
    $output=get_post($postid);
    //var_dump($output);
    //标题
    $title=$output->post_title;
    //缩略图
    $large_image_url = wp_get_attachment_image_src(get_post_thumbnail_id($postid),"medium");
    //var_dump($large_image_url[0]);
    if($large_image_url[0]!="")
    {
        //$file="/var/www/html/wordpress/wp-content/uploads/2020/11/2020-11-24-202040.png";
        $file=str_replace("https://www.rdonly.com","/var/www/html/wordpress",$large_image_url[0]);
        //var_dump($file);
    }
    if($file==""){
        return ;
    }
    $thumb_media_id=UploadImageToWeixin($token,$file);
    //作者
    $author="Appfuns";
    $digest="";
    $show_cover_pic=0;

    //内容
    // 上传文章中的所有图片 
    $newcontent=GetAllImages($token,$output->post_content);
    //加上网站链接地址
    $link="";
    $custom_fields = get_post_custom($postid);
    $count=0;
    foreach($custom_fields as $keys=>$values){
        if ( '_' == $keys{0} )
            continue;
        else{
            $count++;
        }
    }
    if($count){
        foreach($custom_fields as $keys=>$values){
            if ( '_' == $keys{0} )
                continue;
            foreach($values as $key => $value){
                $link=$link."<p><span style='background-color: rgb(0, 0, 0);color: rgb(255, 255, 255);'><strong>".$value."</strong></span></p>";
            }
        }
    }

    //var_dump($link);
    $content=$newcontent.$link;
    
    //原文地址
    $content_source_url=$output->guid;
    $need_open_comment=0;
    $only_fans_can_comment=0;

    $arrcontent=array();
    $arrcontent["title"]=$title;
    $arrcontent["thumb_media_id"]=$thumb_media_id;
    $arrcontent["author"]=$author;
    $arrcontent["digest"]=$digest;
    $arrcontent["show_cover_pic"]=$show_cover_pic;
    $arrcontent["content"]=$content;
    $arrcontent["content_source_url"]=$content_source_url;
    $arrcontent["need_open_comment"]=$need_open_comment;
    $arrcontent["only_fans_can_comment"]=$only_fans_can_comment;
    $data=array("articles"=>array($arrcontent));
    // var_dump($data);
    return json_encode($data,JSON_UNESCAPED_UNICODE);
}

//获取文章中的所有图片
function GetAllImages($token,$content){
    preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $content, $matches);
    //var_dump($matches);
    for($i=0;$i<count($matches[1]);$i++){
        $WeiXinSrc=UpImgToWeixinRtnUrl($token,$matches[1][$i]);
        if($WeiXinSrc!=""){
            $content=str_replace($matches[1][$i],$WeiXinSrc,$content);
        }
    }
    return $content;
}

//上传微信返回url
function UpImgToWeixinRtnUrl($token,$file){
    $url="https://api.weixin.qq.com/cgi-bin/media/uploadimg?access_token=".$token;

    if($file!=""){
        $file=str_replace("https://www.rdonly.com","/var/www/html/wordpress",$file);
        //var_dump($file);
        $filedata = array(
            'media'=> new CURLFile($file)
        );
        $curl=curl_init();
        curl_setopt($curl, CURLOPT_URL, $url);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($curl, CURLOPT_POST, 1);
        //curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
        curl_setopt ($curl, CURLOPT_POSTFIELDS, $filedata);
        $output =curl_exec($curl);
        // var_dump($output);
        $result=json_decode($output);
        curl_close($curl);
        var_dump($result);
        return $result->url;
    }else
    {
        return "";
    }
    // var_dump($url);
    // return $result->media_id; 
}

// 上传图片
function UploadImageToWeixin($token,$file){
    //$token=GetWeixinToken();
    //$file="/var/www/html/wordpress/wp-content/uploads/2020/11/2020-11-24-202040.png";
    $url="https://api.weixin.qq.com/cgi-bin/material/add_material?access_token=".$token."&type=thumb";

    $filedata = array(
        'media'=> new CURLFile($file)
    );

    $curl=curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($curl, CURLOPT_POST, 1);
    //curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    curl_setopt ($curl, CURLOPT_POSTFIELDS, $filedata);
    $output =curl_exec($curl);
    // var_dump($output);
    $result=json_decode($output);
    curl_close($curl);
    //var_dump($result->media_id);
    // var_dump($url);
    return $result->media_id; 
}

//调用接口发送数据
function SetWeixinArtical(){
    $token=GetWeixinToken();
    $data=GetArticalInfo($token);
    // var_dump($data);
    // $headers = array("Content-type:text/json;charset=utf-8");
    $url="https://api.weixin.qq.com/cgi-bin/material/add_news?access_token=".$token;
    // var_dump($url);
    $curl=curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($curl, CURLOPT_POST, 1);
    curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    curl_setopt ($curl, CURLOPT_POSTFIELDS, $data);
    $output =curl_exec($curl);
    // //var_dump($output);
    $result=json_decode($output);
    curl_close($curl);
    // //var_dump($result);
    // // var_dump($url);
    // return $result->access_token; 
}
add_action('publish_post','SetWeixinArtical');