From SA-MP Wiki

Use them to work with GetPlayerState or the OnPlayerStateChange callback.

Often used

0 Empty (while initializing) - "PLAYER_STATE_NONE"
1 Player is on foot - "PLAYER_STATE_ONFOOT"
2 Player is driver of a vehicle - "PLAYER_STATE_DRIVER"
3 Player is passenger of a vehicle - "PLAYER_STATE_PASSENGER"
7 Player is wasted or on class selection - "PLAYER_STATE_WASTED"
8 Player is spawned - "PLAYER_STATE_SPAWNED"
9 Player is spectating - "PLAYER_STATE_SPECTATING"

Used internally

4 Player exits a vehicle
5 Player enters a vehicle as driver
6 Player enters a vehicle as passenger