This function looks up images in the HTML email body content that are encoded in Base64, attaches them to the Swift_Message, replaces the Base64 with CID and returns modified body.

swift_mailer_image_replacer.php

protected function replaceBase64ImagesWithCid($content, Swift_Message $message)
{
    return preg_replace_callback(
        "/(<img[^>]*src *= *[\"']?)([^\"']*)/i",
        function ($matches) use ($message) {
            $is_base64 = preg_match("/data:([a-z]+\/[a-z]+);base64,(.*)/i", $matches[2], $image_data);
            if ($is_base64) {
                $contentType = $image_data[1];
                list($type, $ext) = explode("/", $contentType);

                $base64 = $image_data[2];
                $filename = uniqid("image_") . "." . $ext;

                $cid = $message->embed(new Swift_Image(base64_decode($base64), $filename, $contentType));

                return $matches[1] . $cid;
            }
            return $matches[0];
        },
        $content
    );
}

Swift Mailer - Convert Base64 images to CID

This function looks up images in the HTML email body content that are encoded in Base64, attaches them to the Swift_Message, replaces the Base64 with CID and returns modified body.

You just have to provide original content and an instance of Swift_Message to attach the images to.

Sample usage

$message = (new Swift_Message($subject))
            ->setFrom([$from_email => $from_name])
            ->setTo([$to_email]);

$message->setBody($this->replaceBase64ImagesWithCid($content, $message), 'text/html');