I think it is a good idea to use PHP, or at least a subset of, as a template language. It will be easier for new developers to be integrated with your system as there's no special library syntax to learn and you don't have the overhead of that library parsing your template just to turn it into PHP.
Also, you missed a context in which you would need to eliminate the terminating semi-colon in your variable echo statements as well, e.g. <?= $receiver->name ?>. If you're going to not use semi-colons in your templates then you really need to not use them. Clearly your code example doesn't use them but I just wanted to point out the missing context in which not to use terminating semi-colons.
The really important thing is having team buy-in and ensuring consistency across your templates. Personally, I find it jarring to not have the semi-colon...I instinctively want to add those semi-colons into all your PHP statements. Do you have a person on your team with the same level of OCD? How will they react to this? If your team doesn't have buy-in and one or two people are vehemently against it, make them defend their decision, then maybe this isn't the best way to go.
Another potential pitfall would be incorporating new team members. They may very well not realize you can even do this, go back in your templates and insert all the terminating semi-colons where appropriate. Obviously this is a case of a developer not knowing as much about PHP as they should but it is something that you'd want to weigh in your decision. Educating new, junior developers that in templates we don't use terminating semi-colons but other code we do.
tl;dr
Get buy-in from your team, ensure your templates are consistent and have a reasonable plan to educate new developers on how PHP code in templates differ from "normal" PHP code (alternative syntax, no terminating semi-colon, allowed PHP structures, etc).
