Testing is one of the most important aspects of the application development. While being an additional cost, it saves a lot of stress and improves the stability. It also greatly lowers the chances of other issues not being checked "because that was only small change" and noone made sure they're still OK.
Unit tests and Integration tests can be implemented by us with your application. We are most often using solutions such as PHPUnit and Selenium, but we can also use others! Testing solutions depend on your application and the whole flow, really.
Regression tests can be added to make sure that no error reappears, which is very often a problem in no-testing approach and active development.
Manual testing is usually left in client's hands during the client acceptance process, but is albo possible to be done on our end with reports sent on what was tested and spotted & fixed.
Our testing process also improves the UX in the process as an effect. We always try to make tests being the simplest possible, and we spot user experience problems during the test making process thanks to that approach. (which is additional gain for the same price!)
While being an additional cost to "no testing" approach - in the end - you'll know that implementing the tests was definitely worth it.