SetTimerEx

SetTimerEx("EndAntiSpawnKill", 5000, false, "i", playerid);
// EndAntiSpawnKill - The function that will be called
// 5000 - 5000 MS (5 seconds). This is the interval. The timer will be called after 5 seconds.
// false - Not repeating. Will only be called once.
// "i" - I stands for integer (whole number). We are passing an integer (a player ID) to the function.
// playerid - The value to pass. This is the integer specified in the previous parameter.

// The event callback (OnPlayerSpawn) - we will start a timer here
public OnPlayerSpawn(playerid)
{
    // Anti-Spawnkill (5 seconds)

    // Set their health very high so they can't be killed
    SetPlayerHealth(playerid, 999999.0);

    // Notify them
    SendClientMessage(playerid, -1, "You are protected against spawn-killing for 5 seconds.");

    // Start a 5 second timer to end the anti-spawnkill
    SetTimerEx("EndAntiSpawnKill", 5000, false, "i", playerid);
}

// Forward (make public) the function so the server can 'see' it
forward EndAntiSpawnKill(playerid);
// The timer function - the code to be executed when the timer is called goes here
public EndAntiSpawnKill(playerid)
{
    // 5 seconds has passed, so let's set their health back to 100
    SetPlayerHealth(playerid, 100.0);

    // Let's notify them also
    SendClientMessage(playerid, -1, "You are no longer protected against spawn-killing.");
    return 1;
}