[Mean]

pawn Код:

forward IsPlayerInZone ( playerid, zoneid );
public IsPlayerInZone ( playerid, zoneid ) // PUBLIC!!!
{
    new TmpZone[ MAX_ZONE_NAME ];
    GetPlayer3DZone ( playerid, TmpZone, sizeof ( TmpZone ) );
    for ( new i = 0; i != sizeof(gSAZones); i++ )
    {
        if ( strfind ( TmpZone, zone, true ) != -1 )
        return 1;
    }
    return 0;
}

Re-Written
If you want to use a forward, you need to make that stock as a public!
Tip for the future