[xines]

Mixed this together, give it a try.

Spoiler

PHP Code:

#include <sourcemod>
#include <store>

public void OnPluginStart()
{
    HookEvent("round_start", Event_RoundStart);
}

public Action Event_RoundStart(Event event, const char[] name, bool dontBroadcast)
{
    static int roundcount;
    if(++roundcount >= 5)
    {
        int randomclient = GetRandomClient();
        if(IsValidClient(randomclient))
        {
            int randomcredits = GetRandomInt(1, 100);
            Store_SetClientCredits(randomclient, Store_GetClientCredits(randomclient) + randomcredits);
            PrintToChatAll("%N got %i random credits!", randomclient, randomcredits);
        }
        roundcount = 0;
    }
    
    return Plugin_Continue;
}

int GetRandomClient()
{
    //Clear variables 
    int iCount = 0;
    int[] clients = new int[MaxClients+1];
    
    //Do loop and return selected client
    for (int i = 1; i <= MaxClients; i++)
        if(IsValidClient(i)) clients[iCount++] = i;
    
    return (iCount == 0) ? -1 : clients[GetRandomInt(0, iCount-1)];
}

stock bool IsValidClient(int client)
{
    return (1 <= client <= MaxClients && IsClientInGame(client));
}