Heroku deployment doesn't work

[ham7a]

Deploying on Heroku returns the following error:
There was an issue building your app. This can mean your app.json's project is not a valid Heroku application. Please ensure your app is deployable to Heroku and try again.

The log shows the following:
Preparing runtime environment...
-----> Checking for additional extensions to install...
-----> PHP app detected
! ERROR: Failed to resolve runtime requirements.
If the issue persists, please contact support.
! Push rejected, failed to compile PHP app.
! Push failed

Any solution?