[Scenario]

pawn Code:

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if(newkeys & KEY_SECONDARY_ATTACK)
    {
        if(EnteredInterior[playerid]) return cmd_exit(playerid, "");
        else return cmd_enter(playerid, "");
    }
    return 1;
}

The above code will basically call /enter or /exit depending on the validity of this bool statement:

pawn Code:

EnteredInterior[playerid]

You will want to make this a true statement in your /enter command, so towards the bottom or something, do this:

pawn Code:

EnteredInterior[playerid] = true;

... and you'll want to make this a false statement in your /exit command, so towards the bottom or something, add this:

pawn Code:

EnteredInterior[playerid] = false;

Don't forget to add this at the top of your script...

pawn Code:

new bool:EnteredInterior[MAX_PLAYERS] = false;