How to fetch Email addres of an author via ID?

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I want to make an HTML form which will have an input field to fill in the ID of the user. Then I want to pass the value into a php code such that the email address associated with that user ID is displayed. However, I am new to WP and don't know how to do it correctly. I made a new php file in the root folder of my WP directory and put in this code and tried by browsing the file, but it didn't…

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