public OnPlayerUpdate(playerid)
{
    new iCurWeap = GetPlayerWeapon(playerid); // Return the player's current weapon     
    if(iCurWeap != GetPVarInt(playerid, "iCurrentWeapon")) // If he changed weapons since the last update
    {
        // Lets call a callback named OnPlayerChangeWeapon
        OnPlayerChangeWeapon(playerid, GetPVarInt(playerid, "iCurrentWeapon"), iCurWeap);
        SetPVarInt(playerid, "iCurrentWeapon", iCurWeap);//Update the weapon variable
    }
    return 1; // Send this update to other players.
}
 
stock OnPlayerChangeWeapon(playerid, oldweapon, newweapon)
{
    new     s[128],
        oWeapon[24],
        nWeapon[24];
 
    GetWeaponName(oldweapon, oWeapon, sizeof(oWeapon));
    GetWeaponName(newweapon, nWeapon, sizeof(nWeapon));
 
    format(s, sizeof(s), "You changed weapon from %s to %s!", oWeapon, nWeapon);
 
    SendClientMessage(playerid, 0xFFFFFFFF, s);
}