微信文档地址 https://developers.weixin.qq.com/miniprogram/dev/api-backend/open-api/qr-code/wxacode.getUnlimited.html

第一步需要获取 ACCESS_TOKEN

public static function getAppletAccessToken()
{
    //获取access_token
    $access_token = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid=$APPID&secret=$APPSECRET";
    $json =$this->httpRequest( $access_token );
    $json = json_decode($json,true);
    $ACCESS_TOKEN = $json["access_token"];
    return $ACCESS_TOKEN;
}

/**
 * @param $url
 * @param string $data
 * @param string $method
 * @return mixed
 * curl_init 请求
 */

public function httpRequest($url, $data='', $method='GET')
{
    $curl = curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, 0);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($curl, CURLOPT_USERAGENT, $_SERVER['HTTP_USER_AGENT']);
    curl_setopt($curl, CURLOPT_FOLLOWLOCATION, 1);
    curl_setopt($curl, CURLOPT_AUTOREFERER, 1);
    if($method=='POST')
    {
        curl_setopt($curl, CURLOPT_POST, 1);
        if ($data != '') {
            curl_setopt($curl, CURLOPT_POSTFIELDS, $data);
        }
    }
    curl_setopt($curl, CURLOPT_TIMEOUT, 30);
    curl_setopt($curl, CURLOPT_HEADER, 0);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
    $result = curl_exec($curl);
    curl_close($curl);
    return $result;
    
}

第二步 生成二维码

public function actionInviteFriends()
{
        header('content-type:text/html;charset=utf-8');
        $ACCESS_TOKEN =$this->getAppletAccessToken();
        //构建请求二维码参数
        $url ="https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=$ACCESS_TOKEN";
        $scene = $invite_code;
        $page = "";
        $param = json_encode(['scene'=>$scene,'page'=>$page]);
        //POST参数
      $result =$this->httpRequest( $url, $param,"POST");
        //保存图片-生成二维
      file_put_contents("qrcode.png", $result);
        $base64_image ="data:image/jpeg;base64,".base64_encode( $result );
       
        return json(['code'=>1,'data'=>$base64_image ]);
  
}

3，微信的 ACCESS_TOKEN 是有上限的，有效时间 2小时。实际项目中需要做一个缓存，防止量大