CakePHP: upgrade from 3.6.x to 4.0 "Missing Template Exception" - php

Question

I followed the upgrade guide (after I upgraded from 3.6 to 3.8) but I get this error:
Error: [Cake\View\Exception\MissingTemplateException] Template file "Error\error500.ctp" is missing
....
If you want to customize this error message, create src\Template\Error\fatal_error.ctp
After the upgrade procedure my templates moved to app_name\templates directory and renamed to *.php instead of *.ctp . I updated the app.php and app.default.php with the new paths:
'App' => [
'namespace' => 'App',
'encoding' => env('APP_ENCODING', 'UTF-8'),
'defaultLocale' => env('APP_DEFAULT_LOCALE', 'en_US'),
'defaultTimezone' => env('APP_DEFAULT_TIMEZONE', 'UTC'),
'base' => false,
'dir' => 'src',
'webroot' => 'webroot',
'wwwRoot' => WWW_ROOT,
//'baseUrl' => env('SCRIPT_NAME'),
'fullBaseUrl' => false,
'imageBaseUrl' => 'img/',
'cssBaseUrl' => 'css/',
'jsBaseUrl' => 'js/',
'paths' => [
'plugins' => [ROOT . DS . 'plugins' . DS],
'templates' => [ROOT . DS . 'templates' . DS],
'locales' => [ROOT . DS . 'Locale' . DS],
],
],
But still the application is looking for the template files with .ctp extension and under src\Template\...
What am I missing?

Answer 1

From my experience:
Upgrade CakePHP 3.x to 4.x
composer update
run phpstan from --level 0 to 2 and fix your code or test in scrutinizer
read 4.0 Migration Guide : https://book.cakephp.org/4/en/appendices/4-0-migration-guide.html
read 4.0 Upgrade Guide : https://book.cakephp.org/4/en/appendices/4-0-upgrade-guide.html
Steps:
Install the upgrade tool
Rename locale files
Rename template files
Once you've renamed your template and locale files, make sure you update App.paths.locales and App.paths.templates paths to be correct.
From your app composer file remove all cakephp/* (cakephp3) packages, also phpstan, code standards, phpunit,.. run composer update
Applying Rector Refactorings
composer require --update-with-dependencies "phpunit/phpunit:^8.0"
composer require --update-with-dependencies "cakephp/cakephp:4.0.*"
Install fresh cakephp4 inside your app folder. Example: mkdir cakephp4 && cd cakeph4; and run composer create-project --prefer-dist cakephp/app:4.* .;
compare your old cakephp files with files from cakephp4 folder, update all and copy missing files
delete cakephp4 and upgrade folders
don't forget at top of you php files to add <?php declare(strict_types=1);
composer cs-check then composer cs-fix