I thought that I would like to publish a portfolio site created with php (using MySQL, phpMyAdmin, MAMP) on Heroku, and it was smooth until I created an application and connected it to Github. However, when the application file connected with Github is "Deploy branch" with "Manual deploy", the following error message appears and it is stuck. (All operations are on the browser.)

I was proceeding while referring to the following article.
heroku beginner-try deploying from GitHub-

Error message
----->App not compatible with buildpack: https://buildpack-registry.s3.amazonaws.com/buildpacks/heroku/php.tgz
 ! ERROR: Application not supported by this buildpack!
 The 'heroku/php' buildpack is set on this application, but was
 ! unable to detect a PHP codebase.
 ! A PHP app on Heroku requires a 'composer.json' at the root of
 the directory structure, or an 'index.php' for legacy behavior.
 If i am trying to deploy a PHP application, ensure that one
 of these files is present at the top level directory.
 ! If i am trying to deploy an application written in another
 ! language, you need to change the list of buildpacks set on your
 ! Heroku app using the 'heroku buildpacks' command.
 For more information, refer to the following documentation:
 ! https://devcenter.heroku.com/articles/buildpacks
 ! https://devcenter.heroku.com/articles/php-support#activation
       More info: https://devcenter.heroku.com/articles/buildpacks#detection-failure
 ! Push failed

When you read the error text, you need a "composer.json" file to publish a site created with php on Heroku. I put it in.

    "require": {
      "php": "~ 7.0",
      "ext-mbstring": "*"

The "index.php" file is included.

I tried to deploy again after doing the above, but I still get the same error.
Is there anything else I need?
I'm new to Heroku and I'm not sure how to deal with it.