Manual:ConvertExtensionToRegistration.php

Details
convertExtensionToRegistration file is a maintenance script, which helps you migrating skins and extensions from PHP entry points to a JSON metadata file. If your extension supports older versions of MediaWiki, you should keep your PHP entry point FooBar/FooBar.php until you drop support for those older versions

Examples
php maintenance/convertExtensionToRegistration.php extensions/BugtrackerStatus/BugtrackerStatus.php

This writes the variable dclarations of the entry point BugtrackerStatus.php into an extension.json file. Afterwards, you might still want to manually clean up these declarations inside the BugtrackerStatus.php file and add a call to wfLoadExtension as detailed on Manual:Extension_registration.

Options
Usage: php convertExtensionToRegistration.php [ ]

Arguments: Location to the PHP entry point you wish to convert

Options: --skin     Whether to write to skin.json; use this option for converting a skin!