If you are working with composer you may also need to work with a specific version of php. While you can edit composer.json it is easier to specify when you call composer...
As any of our customers will know Plesk has always provided 1 click PHP selection on a per domain basis. This is a powerful tool for anyone rolling out different applications or testing upgrades with newer version of PHP. However, Ubuntu 18 supports only those versions of PHP that support OpenSSL 1.1, and that are PHP 7.x and newer. Therefore, if you require php 5.6 then you will need to take additional steps and manually install PHP 5.6 and add this as an additional handler in Plesk.
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How to add www retrospectively to plesk domain.
I have added this post due to the wide range of conflicting information that is currently on the web around this matter. We have seen people who should know better telling people that it is not possible and others just giving plain wrong explanations.