How to get a sign's position

How do I get a sign's position and save it to a config file?

Do you mean player place a sign?

No, I am trying to create a shop sign plugin for my server to use with my custom economy plugin, but I am not sure how to get a sign's position/coords.

When do you want to save it?(Event)

PHP:

/** @var \pocketmine\tile\Sign $sign */
$x = $sign->getX();
$y = $sign->getY();
$z = $sign->getZ();
$config->set($pathToYourEntry, [$x, $y, $z]);

I thought you couldn't define $config.

You can. All variables can be defined. In this case, $config refers to the expression that resolves as your Config object, and it can be a variable called $config or whatever you like.
For example:

PHP:

$config = $this->getConfig();
$config->set("a", "b");


Does the same thing as:

PHP:

$this->getConfig()->set("a", "b");

Would u want to save it when a player places a sign (SignPlaceEvent) or when a player interacts with a sign (PlayerInteractEvent)?

Actualy, you don't even need to get the sign's tile object to get its coords. A Block (pocketmine\block\Sign) can already represent the position.