Agreed, full rewrite is a horrible idea. Source: worked on a rewrite of a project that was like this: PHP from 2003, 7 figures in revenue, written by someone who was not a developer, no version control or testing. And it failed horribly.

I have tactical suggestions, but the strategy is simple: move toward more modern software practices, one step at a time.

But first, the elephant in the room. You say you need to help the project

> without managing [the team] directly

Who does? How can you help them?

Because you don't have direct authority, all the tactics and suggestions mentioned here won't be as helpful as they would if you were the manager in charge. And it's hard to offer concrete advice without knowing exactly how you are connected. A principal in the same company and want to help? A peer of the manager? A peer of the team members? Each of these would have different approaches.

And how much time do you have to help? Is this something you are doing in the shadows? Part of your job? Your entire job?

With that said, here's my list of what to try to influence the team to implement. Don't worry about best of breed for the tools, just pick what the company uses. If the tool isn't in use at the company, pick something you and the team are familiar with. If there is nothing in that set, pick the industry standard (which I try to supply).

1. version control. Git if you don't have any existing solution. GitHub or GitLab are great places to store your git repos

2. bug tracker. You have to have a place to keep track of issues. GitHub issues is adequate, but there are a ton of options. This would be an awesome place to try to get buy-in from the team about whichever one they like, because the truth it is doesn't matter which particular bug tracker you use, just that you use one.

3. a build tool so you have one click deploys. A SaaS tool like CircleCI, GitHub actions is fine. If you require "on prem", Jenkins is a fine place to start. But you want to be able to deploy quickly.

4. a staging environment. This is a great place to manually test things and debug issues without affecting production. Building this will also give you confidence that you understand how the system is deployed, and can wrap that into the build tool config.

5. testing. As the parent comment mentions, end to end testing can give you so much confidence. It can be easy to get overwhelmed when adding testing to an existing large, crufty codebase. I'd focus on two things: unit testing some of the weird logic; this is a relatively quick win. And setting up at least 1-2 end to end tests through core flows (login, purchase path, etc). In my experience, setting up the first one of each of these is the toughest, then it gets progressively easier. I don't know what the industry standard for unit testing in php is any more, but have used phpunit in the past. Not sure about end to end testing either.

6. Documentation. This might be higher, depending on what your relationship with the team is, but few teams will say no to someone helping out with doc. You can document high level arch, deployment processes, key APIs, interfaces, data stores, and more. Capture this in google docs or a wiki.

7. data migrations. Having some way to automatically roll database changes forward and back is a huge help for moving faster. This looks like a viable PHP option: https://laravel.com/docs/9.x/migrations which might let you also introduce a framework "via the side door". This is last because it is least important and possibly more intrusive.

None of these are about changing the code (except maybe the last one), but they all wrap the code in a blanket of safety. There's the added bonus that it might not trigger sensitivities of the team because you aren't touching "their code". After implementing, the team should be able to move faster and with more confidence.

Since you are not the direct manager, you want to help the team get better through your influence and through small steps. That will build trust and allow you to suggest bigger ones, such as bringing in a framework or building abstraction layers.