tjlytle
Nexmo should be pretty good across the board as it pertains to what delivers best (direct to carrier model: http://blog.nexmo.com/post/4690069179/directsms). In that way, Nexmo is trying to solve the issue you're having.
Have you had any issues with deliverability and Nexmo (where another provider delivered better)? Nexmo actually provides some tools to automatically optimize traffic based on client conversion ratios.
Just a few quick thoughts:
PHP is - in most cases - very tightly coupled to the HTTP request. This can be good or bad depending on what you're building. Doing a lot of offline processing? Probably bad. Serving up web pages? Probably good.
There's no (or little) state with PHP (you do have the session, which can be pushed to memcache), so - generally speaking - running your application across a bunch of web nodes isn't too hard to implement (even scaling the horrible legacy style code that helps give PHP a bad reputation shouldn't be too hard). This may or may not be important to you.
There's really no one true way with PHP - it can be used for a simple 10 line script, or for a large scale web application. If you like (or need) that kind of flexibility, it could be a good choice.
Bottom line is - it's certainly sill worth considering, just know why you picked it (or whatever other language/platform you use).
That said "it's what I know" or "I wanted to try something new" aren't necessarily bad reasons.
Since it's backend as a service, allowing access to view/edit/troubleshoot the data is key.
Also, CloudMailin's (http://www.cloudmailin.com/) 'Try it Live!' is a pretty nice demo of the API.
I thought it did a good job filling the gap between 'how to write automated tests' and actually writing tests for real world code.