直接上代码。

前端代码：
var bgsrc = "img1.png";
var src = "img2.png";
drawAndShareImage(bgsrc, src);

  function drawAndShareImage(bgsrc, src) {
        let canvas = document.createElement('canvas');
        canvas.width = 300;
        canvas.height = 531;
        let context = canvas.getContext('2d');
        context.rect(0, 0, canvas.width, canvas.height);
        let bgImg = new Image();
        bgImg.src = bgsrc;    // 背景图的url
        bgImg.crossOrigin = 'Anonymous';
        bgImg.onload = () => {
            context.drawImage(bgImg, 0, 0, 300, 531);
            let img = new Image();
            img.src = src;    // 需要合进去的图片url
            img.crossOrigin = 'Anonymous';
            img.onload = () => {
                context.drawImage(img, 66, 231, 168, 168);
                let base64 = canvas.toDataURL('png');
                // 发给后端，后端保存并可返回地址到前端再做显示。
                $.post("{WEB_PATH}/mobile/ajax/sharesimg/", {img: base64}, function (data) {
                    // Do something
                 });
                 //canvas直接将 base64 图片显示到img标签
                //  $(".shareImg img").attr("src", base64);
            }
        }
    }

后端代码（具体根据你自己业务逻辑更改）：

public function sharesimg()
{
    if (!$this->userinfo) {
        echo 0;
        die;
    }
    $member = $this->userinfo;

    $img = isset($_POST['img']) ? rawurldecode($_POST['img']) : '';

    if (empty($img)) {
        echo 0;
        die;
    }
    $uid = $this->userinfo['uid'];
    $memberinfo = $this->db->GetOne("select * from `@#_member` where uid='$uid'");

    if (empty($memberinfo['sharesimg'])) {
    //匹配出图片的格式
            if (preg_match('/^(data:\s*image\/(\w+);base64,)/', $img, $result)) {
                $type = $result[2];
                $filename = 'sharesimg/' . time() . ".jpg";
                $new_file = dirname($_SERVER['SCRIPT_FILENAME']) . '/data/uploads/' . $filename;
                $base64_body = substr(strstr($img, ','), 1);
                if (file_put_contents($new_file, base64_decode(str_replace($result[1], '', $img)))) {
                    
                    $sql = "update `@#_member` set sharesimg='$filename' where uid='$uid'";
                    if ($this->db->Query($sql)) {
                        $tname = $filename;
                        if (file_exists($new_file)) {
                            $wh = getimagesize($new_file);
                            $x = 0;
                            $y = 0;
                            $w = $wh[0];
                            $h = $wh[1];
                            $point = array("x" => $x, "y" => $y, "w" => $w, "h" => $h);
    
                            System::load_sys_class('upload', 'sys', 'no');
                            upload::thumbs(160, 160, false, G_UPLOAD . $tname, $point);
                            upload::thumbs(80, 80, false, G_UPLOAD . $tname, $point);
                            upload::thumbs(30, 30, false, G_UPLOAD . $tname, $point);
                        }
                        echo $filename;
                    } else {
                        echo 0;
                    }
                } else {
                    echo 0;
                }
            } else {
                echo 0;
            }
    } else {
        echo $memberinfo['sharesimg'];
    }

}