Spawning in vicinity?

Is there a script which I can refer to that spawns gameobjects (enemies, in this case zombies) from spawners that work only when the player is within the vicinity of the spawner, and despawns them the moment your distance from the spawner is too large.

Like that example. I'm relatively new to scripting and so I needed the base script to which I would add more parameters when deciding who to spawn (if it's raining or not, and if there is sunlight or not).

Thank you for your time!