From the urls of the emojis I deduct phpBB is using Emojione 3.1. (cdn.jsdelivr.net/emojione/assets/3.1/) They state in their Free license agreement that the emojis cannot be used in open source projects (see 3.4.F)
:heart: -> -> the image location is https://cdn.jsdelivr.net/emojione/assets/3.1/png/64/2764.png
Btw. for Emojione 2.2.7: https://github.com/emojione/emojione/tree/2.2.7 : In general, proper attribution/credit must be given on every web page, app, or video description where our emojis are displayed.