I've installed Fontawesome using bower, and I ran npm run dev. I can use any of the Fontawesome free brand icons like Quora, Paypal, Amillia, but when I try to use the PHP icon nothing is displayed. Any ideas?
Fontawesome recently merged to their new version 5 but installing them through package managers still pulls in the older version. You are probably using their current website to copy the icon html tags, but the code you have expects the old tags.
An easy way to check if this is the case is to install their CDN on the head of your page and see if the icons work.