Hi Thomas 
The user’s avatar URL from Discourse is:
https://sea2.discourse-cdn.com/infiniteflight/user_avatar/community.infiniteflight.com/legendaryroro88/120/704016_2.png
The output of your link, including what your PHP code returns, is:
https://sea2.discourse-cdn.com/infiniteflight/user_avatar/community.infiniteflight.com/LegendaryRoro88/120/352328_2.png
Spot the difference and it will help you solve your problem. 