Ersatz

Ersatz allows you to use and create PlaceholderAPI placeholders within Skript

Docs:
https://skripthub.net/docs/?addon=Ersatz

Example usage:

Making custom placeholders:

on placeholder request with prefix "custom":
  if the identifier is "hey": # %custom_hey%
    set the result to "hi there %player%!"
  else if the identifier is "bye": # %custom_bye%
    set the result to "bye bye %player%"

Getting the value of a placeholder:

on join:
  set {_ping} to placeholder "player_ping" # placeholder "%%player_ping%%" is also valid
  broadcast "your ping is %{_ping}%"