Now i’ll share a Module to alter the existing Drupal “Site Information” form. Specifics:

• A new form text field named “Site API Key” needs to be added to the “Site Information” form with the default value of “No API Key yet”.
• When this form is submitted, the value that the user entered for this field should be saved as the system variable named “siteapikey”.
• A message should inform the user that the Site API Key has been saved with that value.
• When this form is visited after the “Site API Key” is saved, the field should be populated with the correct value.
• The text of the “Save configuration” button should change to “Update Configuration”.
• This module also provides a URL that responds with a JSON representation of a given node with the content type “page” only if the previously submitted API Key and a node id (nid) of an appropriate node are present, otherwise it will respond with “access denied”.

This Drupal Module is available here at GitHub.
https://github.com/nirmalyamondal/Drupal-Modules/tree/master/site_apikey