[jasperschellekens]

What you have done wrong is this:
new superadmin;

The line above you used, this isn't a player but a global one.
So it should be this:

new superadmin[MAX_PLAYERS];

So this would be a correct check:

PHP код:

stock IsPlayerSuperAdmin(playerid)
{
 if(superadmin[playerid]==1){
      return 1;
    }
  return 0;
} 


About your login:

PHP код:

CMD:login(playerid, params[]){
    new password;
    if(sscanf(params, "i",password))return SendClientMessage(playerid, 0x33AA33AA, "USAGE: /login [PASSWORD]");
    if(password != blablabla) return SendClientMessage(playerid, 0x33AA33AA, "Wrong Password.");
    superadmin=1;
    SendClientMessage(playerid, 0x33AA33AA, "You have logged in.");
    return 1;
    } 


superadmin=1; sets only one var while in this case we want to use superadmin[playerid]=1;

Also don't forget to empty the it on connect or disconnect otherwise when ID5 admin leaves, the next ID5 is still admin.
superadmin[playerid]=0;