Is PHP echo $str1, $str2, $str3 faster than echo $str1 . $str2 . $str3?
Yes, but only marginally so, and probably won't impact your web application. Here's one of the many articles that runs through the benchmarks.
The result shows that dot is more preferable if there are no variables or $ symbol involved which is around 200% faster. On the other hand, commas will help to increase around 20%-35% efficiency when dealing with $ symbols.