Place code in init.sqf:
// Functions
fnc_function =
{
DEBUG = DEBUG + [_this];
};

// Initiation
enableSaving [false, false];
DEBUG = [];

// Code
["This works for server, not for (random?) clients.", "fnc_function", true] call BIS_fnc_MP;
["This works for both clients and server.", "fnc_function", true, true] call BIS_fnc_MP;

sleep 1;
["This works for both clients and server.", "fnc_function", true] call BIS_fnc_MP;
["This works for both clients and server.", "fnc_function", true, true] call BIS_fnc_MP;

Server's DEBUG: ["This works for server, not for (random?) clients.","This works for both clients and server.","This works for server, not for (random?) clients.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server."]
Client's DEBUG: ["This works for both clients and server.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server.","This works for both clients and server."]