How do I split code and include when I'm developing a plugin?

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Dear friends, I greet you! I recently started working with Grav CMS. I will be glad if someone will support me.

How do I split code and include when I’m developing a plugin?
I need to process a large array of 1500 key-value pairs. The only thing I came up with is to store it in a separate php file and include it in the plugin.

Questions:

  1. Where to store a php file with a long array and how to include it?
  2. Is there a better way to handle a large array of key-value pairs?