When you use echo you can use the ',' operator instead of the '.' operator.
If you use '.' PHP will concatinate the string first, store it in a temp variable and then ouput it using echo.
If you use a comma seperated list there is no such concatination and it simply pushes the values one at a time to the output.
This is a nice little tip I really wanted to tell people about as very few seem to use it.

Keep up the great work!