DKIM Signing Outgoing Mails from CakePHP app

Hello All,
I have a small community mailer app built in CakePHP 2.8.0 which is running on PHP7.0/Debian8/MariadB 10.1
If I shot a test mail using PHP Mail(), the mail is correctly signed with a DKIM signature, so the configuration at server level is working and good. However, outgoing mails generated through the CakePHP do not to even have DKIM in their header.

I tried to find but could not get much results… How do you make outgoing mails generated through CakePHP to be DKIM signed? What necessary config is to be done so that DKIM signature be injected into the mail headers.

It’s interesting. I use MTA for that, but I found this article:

May be it helps.