Yii2 App Engine Standard AssetManager. Must rename web/ -> public/ or specify an entrypoint as entrypoint: web/index.php in your app.yaml file

app.yaml

runtime: php72
handlers:
- url: /css
  static_dir: public/css

- url: .*
  script: public/index.php

AssetManager.php

<?php

namespace app\components;

use yii\base\InvalidConfigException;
use yii\helpers\FileHelper;


class AssetManager extends \yii\web\AssetManager
{
    /**
     * Initializes the component.
     * @throws InvalidConfigException if [[basePath]] is invalid
     * @throws \yii\base\Exception
     */
    public function init()
    {
        if ($this->basePath) {
          FileHelper::createDirectory($this->basePath);  
        }
    
        parent::init();
    }
    
}

index.php

<?php
// comment out the following two lines when deployed to production


require __DIR__ . '/../vendor/autoload.php';
require __DIR__ . '/../vendor/yiisoft/yii2/Yii.php';

$config = require __DIR__ . '/../config/web.php';

$pathInfo = $_SERVER['REQUEST_URI'];
$assetBaseUrl = $config['components']['assetManager']['baseUrl'];
if (\yii\helpers\StringHelper::startsWith($pathInfo, $assetBaseUrl)) {
    $path = '/tmp' . $pathInfo;

    if (file_exists($path)) {
        $mime = \yii\helpers\FileHelper::getMimeTypeByExtension($path);
        header('Content-Type: ' . $mime);
        echo file_get_contents($path);
    } else {
        throw new \yii\web\NotFoundHttpException();
    }
} else {
    $app = new yii\web\Application($config);
    $app->run();
}

web.php

<?php
$config = [
  ...
  'runtimePath' => '/tmp',
  'components' => [
      'assetManager' => [
          'class' => 'app\components\AssetManager',
          'baseUrl' => '/assets',
          'basePath' => '/tmp/assets'
        ] 
   ],
  
  ....
  // set the baseUrl on the request component to '/' to play nicely with dev_appserver.py
  'request' => [
            // !!! insert a secret key in the following (if it is empty) - this is required by cookie validation
            'cookieValidationKey' => 'JBZIK8HaM3KaOig3KYyzh6GGTBXCNH98',
            'baseUrl' => '/'
        ],
  
  ....
  // must enablePrettyUrl and set showScriptName to false.  Also set baseUrl to '/' to play nicely with dev_appserver.py
  'urlManager' => [
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'baseUrl' => '/',
            'rules' => [
            ],
        ],
];