Classic confusion:
"XXX Tower, YYY, a ZZZ, student pilot, first solo, one POB" duh.
But if you forget to mention it and it's been a long day for the controllers:
"XXX Tower, YYY, a ZZZ, student pilot, first solo, request taxi"
"YYY, XXX, say POB"
"One, obviously"
Or the other classic:
"XXX Tower, YYY, a ZZZ, student pilot, cross country solo, zero POB" (Passengers on Board?)