Hello,
I have created a new Jenkins job 'php-composer-validate' which invokes 'composer.json' on a proposed change and bails out whenever it is faulty.
The change is only triggered on changes that modify composer.json.
To have the job triggered on your repository, you would have to edit Zuul configuration in integration/zuul-config.git and add an entry for the job php-composer-validate
Example for MediaWiki core:
https://gerrit.wikimedia.org/r/#/c/136732/1/layout.yaml
Then ask for review :)