You can use the WixFetch API to access an external service. You’ll have to then expose your own API in your PHP site.
For information on using fetch in Wix, see these resources:
Here are some examples that use fetch to access third-party services:
Send Email with SendGrid REST Interface
Send an email using the SendGrid REST API.
Stripe Payment Processing
Integrate the Stripe Payment processing system into a site. Three levels of user access are demonstrated: visitor, freemium (registered), and premium (paid).
Using the Places API from Google Map Services
Demonstrates geolocation queries using the Google Maps Places API web services. The queries are performed in backend (server-side) web modules.