managing multiple application on single codeigniter installation

Just because a framework supports it doesn't mean it's a good solution.

Once upon a time, shared hosting had a limit on the occupied space and the number of files.
In this case, such an approach would be relevant, but now there are no such restrictions. Therefore, Symfony abandoned such a feature.

In addition, using the composer, this can lead to conflicting versions of packages. And when updating the framework, you will have to update all applications.

This approach is a chance to shoot yourself in the foot.

Yes, you are right. I am just playing. Its not for production purpose.

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It is officially supported.
https://codeigniter.com/user_guide/gener...stallation

What's wrong with it?

Its solved. There were some problem on hosting side.