Have you considered one of the (excellent) email plugins?
I highly recommend POP3it Pro and SMTPit Pro from CNS plus-ins:
Peace, love & brown rice,
FileMaker + Web: Design, Develop & Deploy
Certifications: FileMaker 9, 10, 11 & 12
Member: FileMaker Business Alliance
One Part Harmony <http://www.onepartharmony.com/>
Austin, Texas • USA
I've looked at them all, but (having not successfully implemented HTML email with fmPro before, but, having done so with ColdFusion) they seem to require so many tweaks that it's just has hard as writing an interface to a PHP webpage using HTTPpost and far more likely to require handcuffed limitations and breakage with upgrades.
If I'm right in my assessment, and fmPro seems committed to not making html email native nor wysiwyg support for html docs that can be stored in fields, it APPEARS to be potato potahto effort up front and far easier to see scalability and a future using PHP.
I could save a bunch of php templates to the web server and just call them programmatically, not worrying about upgrade breakage and costs/
Am I making the plugin thing harder that it is?
The php script to send email isn't a bad idea, and may suite your needs. A few areas to consider when using a web script as compared to a plugin:
1. Adding attachments, especially PDFs generated on the fly.
2. Network latency for sending multiple emails, waiting for a response to see if the email was sent.
3. Load on SMTP server to sending lots of individual emails. A plugin can give you the option of opening one SMTP connection and sending lots of emails at once, then closing the connection. This greatly decreases the time to send... if you're sending enough email, that can add up to hours or even days of saved time.
My product Mail List Maker uses a plugin for sending and receiving email (in concert with web services we wrote to provide additional functionality) so this is something I've tried to think out as much as possible.
I still like the php script idea you have in mind, and have used it myself on several projects, where it fit in.
Hope this helps.