@wb , Based on your requirement, you could use Azure Logic Apps or SendGrid.
This guide demonstrates how to perform common programming tasks with the SendGrid email service on Azure. The samples are written in PHP. The scenarios covered include constructing email, sending email, and adding attachments.
How to Use the SendGrid Email Service from PHP
You could send emails with Gmail from your App Service app by using Azure Logic Apps. Logic Apps brings a lot more power to your App Service app without adding complexity to your code.
Tutorial: Send email and invoke other business processes from App Service
Refer this doc -Recommended method of sending email
Also, see this information - This is a Proof of Concept to send email using the PHP Mailer library and Office 365.
Send email on App Service using Office 365 (O365)