TL;DR: PHP doesn't care about what MTA you're using.
Longer explanation: this goes way back almost as far as the POSIX standards themselves, but every properly written MTA will provide a binary named
sendmail that will behave exactly as the "official"
sendmail program would be expected to behave.
As a result, every unix program or daemon that, for one reason or other, finds itself needing to email someone, knows they can just call
/usr/sbin/sendmail with known options, and be confident that whatever MTA was installed will know what to do with the message from there on in.
As such, unless you use a specific SMTP PHP module and explicitly to use different mail settings (generally, a remote server/port with or without TLS and/or authentication), it will just call
/usr/sbin/sendmail and let the underlying distribution worry about what happens next.
If your mail isn't arriving, I recommend you check the error logs of the MTA (usually in
/var/log/mail.* but depends on your distribution and MTA) for answers.