Here are some install notes from PHPUnit and an article. Did you install phpunit via composer? Or did you install it another way?
Here are some install directions, hopefully one matches your use case.
PhpUnit docs point towards editing a phpunit file and setting the path to the php binary directly
This article points towards installing PhpUnit with PECL and shows taking a setp to change the PHP executable you'll be using
This SO answer points towards using Composer, but setting some specific config