CMD:changename(playerid, params[])
{
    new name[24], id;
    if(sscanf(params,"ds",id,name)) return SendClientMessage(playerid, -1, "USAGE: /changename [id] [name]");
    SetPlayerName(id, name);
    return 1;
}

CMD:changename(playerid, params[])
{
    new name[MAX_PLAYER_NAME]; // declaring array that we will use to store the input name of the player
    if(sscanf(params,"s[MAX_PLAYER_NAME]",name)) //input check if the player put the correct input see sscanf documentation for further details
    {
        return SendClientMessage(playerid, -1, "USAGE: /changename [name]"); // this will print if the player's input exceeds the MAX_PLAYER_NAME
    }
    new pname[MAX_PLAYER_NAME];
    for(new i;i!=MAX_PLAYERS;i++) // looping through all players to check if the name is already taken
    {
        if(IsPlayerConnected(i)) // checking if the player is online to avoid crashing the server
        {
            GetPlayerName(i,pname,sizeof(pname));// getting the name of the player currently lopped
            if(strcmp(name,pname,true)==0) // compring the input name and the name of the player currently looped
            {
                return SendClientMessage(playerid,-1,"name already taken");
            }
        }
    }
    new msg[128];
    SetPlayerName(playerid,name); // setting the input name as the players name
    format(msg,sizeof(msg)," Your name successfully change to %s",name);
    SendClientMessage(playerid,-1,msg);
    return 1;
}

if(sscanf(params,"ds[24]",id,name))...

CMD:changename(playerid, params[])
{
    new newname[MAX_PLAYER_NAME];
    if(sscanf(params, "s[MAX_PLAYER_NAME]", newname)) return SendClientMessage(playerid, 0xFFFFFFFF, "Syntax: /changename <newname>");
    new checkname[MAX_PLAYER_NAME];
    new bool:nametaken = false;
    for(new i = 0; i < MAX_PLAYERS; i ++)
    {
        GetPlayerName(i, checkname, sizeof(checkname));
        if(strcmp(checkname, newname, true) == 0)
        {
            nametaken = true;
        }
    }
    if(nametaken == true) return SendClientMessage(playerid, 0xFFFFFFFF, "That name is already taken.");
    SetPlayerName(playerid, newname);
    new string[40];
    format(string, sizeof(string), "Name changed to %s.", newname);
    SendClientMessage(playerid, 0xFFFFFFFF, string);
    return 1;
}

if(strcmp(checkname, newname, true) == 0)

if(!strcmp(checkname, newname, true) == 0)