I run a simple web service, and I need to securely send emails to users via PHP.
Your exact requirements of "securely send emails" are unknown. But none of your proposals offers end to end security, i.e. depending on the path one or many parties can intercept and even modify the mail.
SMTP has only hop-by-hop encryption, which means any MTA on the way will have the mail in clear even if TLS was used to transmit the mail to the MTA.
Also there is no guarantee that TLS is used at all because SMTP transfer starts plain (without encryption) and only gets upgraded optionally to TLS with the STARTTLS command. If this fails or if no STARTTLS is offered by the receiving MTA the mail is usually send plain text (i.e. best effort encryption). There are various ways TLS can fail (bad configuration) or can be made to fail by an attacker and there are also firewalls which simply transparently strip STARTTLS support so that they can look into the mail.
Apart from that the next hop MTA is chosen by the DNS MX record. With DNS spoofing the mail can thus be redirected/intercepted unless the sending MTA is using strict DNSSec, which it usually isn't.