esx_core-esx-framework/server/main.lua

WhiteList       = {}
local hasSqlRun = false

function loadWhiteList(cb)
	Whitelist = {}

	MySQL.Async.fetchAll('SELECT * FROM whitelist', {}, function (identifiers)
		for i=1, #identifiers, 1 do
			table.insert(WhiteList, tostring(identifiers[i].identifier):lower())
		end

		hasSqlRun = true

		if cb ~= nil then
			cb()
		end
	end)
end

AddEventHandler('onMySQLReady', function()
	loadWhiteList()
end)

Citizen.CreateThread(function()
	Citizen.Wait(10000)

	if not hasSqlRun then
		loadWhiteList()
	end
end)

AddEventHandler('playerConnecting', function(name, setCallback, deferrals)
	-- Mark this connection as deferred, this is to prevent problems while checking player identifiers.
	deferrals.defer()

	local _source = source
	
	-- Letting the user know what's going on.
	deferrals.update(_U('whitelist_check'))
	
	-- Needed, not sure why.
	Citizen.Wait(100)

	local whitelisted, kickReason, steamID = false, nil, GetPlayerIdentifiers(_source)[1]

	if #WhiteList == 0 then
		kickReason = _U('whitelist_empty')
	elseif not string.match(steamID, 'steam:1') then
		kickReason = _U('steamid_error')
	else

		for i = 1, #WhiteList, 1 do
			if tostring(WhiteList[i]) == tostring(steamID) then
				whitelisted = true
				break
			end
		end

		if not whitelisted then
			kickReason = _U('not_whitelisted')
		end

	end

	if whitelisted then
		deferrals.done()
	else
		deferrals.done(kickReason)
	end
end)