Getting error when visiting demo app home page

I hope it’s okay to post basic questions like this here?

This is my first look at Yii3 — I have an old Yii1 project that needs rewriting and, as it will probably take me most of next year to complete (working part-time), I’m interested to see if I can skip Yii2 and go with Yii3.

My install seemed to go fine (I followed the instructions at https://github.com/yiisoft/yii-demo) but, when I try to visit the home page, I get:

Yiisoft\Factory\Exceptions\NotFoundException: No definition for Psr\Http\Server\RequestHandlerInterface in D:\Apache24\htdocs\yii3\vendor\yiisoft\proxy\src\ProxyTrait.php:20
Stack trace:
#0 D:\Apache24\htdocs\yii3\vendor\yiisoft\yii-debug\src\Proxy\ContainerInterfaceProxy.php(63): Yiisoft\Yii\Debug\Proxy\ContainerInterfaceProxy->repeatError(Object(Yiisoft\Factory\Exceptions\NotFoundException))
#1 D:\Apache24\htdocs\yii3\public\index.php(51): Yiisoft\Yii\Debug\Proxy\ContainerInterfaceProxy->get(‘Yiisoft\Yii\Web…’)
#2 {main}

I’m sure I’m missing some vital step but I can’t figure it out.

Did you run composer install or such?

Yes, the install seemed to go okay, no errors. It’s been a few days since I posted this question, so I ran composer update again just now, but I still got the same error message.

Then, after reading through the issues in GitHub, I noticed one where rolling back to composer v1 rather than v2 seemed to fix a similar problem, so I tried that and now I have the demo running.

The was a bug in one of the dependency packages when running composer version 2.x. As a temporary workaround try downgrading composer to 1.x

By the way, Yii-3 is not yet released.

Alpha is said to arrive at the beginning of 2021.

I was looking at Yii3 to see how different it is to Yii2 and whether or not I would feel comfortable starting a site re-write in Yii3 once it was relatively stable. After a quick look at Yii3, I’m not sure I “get it” the same way I got Yii1 or Yii2 and, for that reason, and the fact that Yii3 will most likely not be released for many months, I’ll probably stick with Yii2 for now.

Correct decision IMHO, and it will likely be easier to transfer your application from Yii-2 to Yii-3 than from Yii-1.