MS SQL provides two primary authentication methods, Windows Authentication or SQL Server Authentication.
Since local MS SQL accounts are problematic to maintain and monitor this option is not allowed in the projects we produce. To my knowledge Microsoft's opinion is that mixed mode on MS SQL should be avoided if possible. (https://docs.microsoft.com/en-us/sql/relational-databases/security/choose-an-authentication-mode?view=sql-server-2017)
Some programming languages like PHP does however not play well with MS SQL AD authentication (especially not cross domain boundaries). So what are the secure alternatives to make PHP application in e.g. a DMZ domain access a MS SQL database in an internal domain? Please consider the problem accross programming languages as PHP is just an example. Are you using middleware to handle the connections between the web application and MS SQL?