As part of code that runs when the application runs.

Always write migrations that can be adopted gradually, and always use transactions where possible.

RIP to those not using Postgres who can’t use DDL in transactions.

command line script method:

1) Get/select the structre/definition for table(s) in given schema/database via comand line script. Exact way to do this varies by database (mysql, pgsql, informix, oracle, etc)

2) upload/check in info from #1 to version control system.

edit per hardwaresofton note about DDL in transactions (way noted below is still a RIP, just less so than 'manual' DDL extraction):

Always old school script(s) to extract/display relevant schema/database DDL changes from transactional log in source control format.

This does assume that full/complete transaction log is available to parse.

Check in the migrations

Migrations, typically via Phinx for PHP

Mysqldump and rdiff-backup