We're using an ancient PHP 3.7.37, and we could be using 4.8 (4.8.21 seems to be newest at time or writing)
Change 266060 abandoned by Hashar:
Update phpunit to 4.8.21
The Jenkins jobs are using 3.7 provided via integration/phpunit which we do not want to bump/maintain See past attempts https://gerrit.wikimedia.org/r/#/q/project:integration/phpunit,n,z . Merely because the version is shared among all MediaWiki release we maintain and some old ones are not supporting PHPUnit 4.x.
Please see T99982 "Upgrade PHPUnit to 4.0 " to bump it up. It is basically blocked on being able to run it via composer.