[Shinja]

In your code you save targetid as "u", it can be a name so no "targetid = INVALID_PLAYER_ID" nor IsPlayerConnected(targetid) can slove it maybe in your sscanf version, "u" works fine with me but try to change "u" to "d"

PHP Code:

CMD:pm(playerid, params[])
{
    new targetid, string[258], tekst[128];
    if(sscanf(params, "ds[128]", targetid, tekst)) return SendClientMessage(playerid, COLOR_WHITE, "{3BB9FF}use: {FFFFFF}/pm  < ID > < tekst >");
    if(targetid == INVALID_PLAYER_ID) return SendClientMessage(playerid, COLOR_RED, "Wrong ID");
    format(string, sizeof(string), "[PM][IN] %s: %s", PlayerName(playerid), tekst);
    SendClientMessage(targetid, COLOR_YELLOW, string);
    format(string, sizeof(string), "[PM][out] %s: %s", PlayerName(targetid), tekst);
    SendClientMessage(playerid, COLOR_RED, string);
    return 1;
}