Manual:ConvertExtensionToRegistration.php

Details
convertExtensionToRegistration.php is a maintenance script, which helps you migrating extensions and skins from PHP entry points to a JSON metadata file named extension.json or skin.json. 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/FooBar/FooBar.php

This detects many variable declarations in the entry point FooBar.php and converts them into settings in an extension.json file. Afterwards, you might still want to manually clean up these declarations inside the FooBar.php file and add a call to  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!