On PHP 8 the sms report test are failing:
1) Piwik\Plugins\Ecommerce\tests\System\EcommerceOrderWithItemsTest::testApi with data set <a href='/43'>#43</a> ('ScheduledReports.generateReport', array('_schedrep_via_sms_one_site', '2011-04-05 00:11:42', array('week'), 'original', 'sms.txt', array(2, 4, 0)))
Piwik\Plugins\Ecommerce\tests\System\EcommerceOrderWithItemsTest: Differences with expected in '/home/travis/build/matomo-org/matomo/plugins/Ecommerce/tests/System/processed/test_ecommerceOrderWithItems_schedrep_via_sms_one_site__ScheduledReports.generateReport_week.original.sms.txt'
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'week April 4 – 10, 2011. 5 Visits (+100%), 16 Actions (+100%), Revenue: $130 (+100%), 5 Goal conversions (+100%), Product Revenue: $130 (+100%), 4 Ecommerce Orders (+100%)'
+'week April 4 – 10, 2011. 5 Visits (+-100%), 16 Actions (+-100%), Revenue: $130 (+-100%), 5 Goal conversions (+-100%), Product Revenue: $130 (+-100%), 4 Ecommerce Orders (+-100%)'
While digging deeper into the failure I realized that the numbers are actually formatted twice. As there are some numeric checks in the sms template we actually can't format the metrics before. That used to work for PHP 7, but it doesn't for PHP 8. Formatting in the template only should be enough nevertheless.
I've tested those changes on another branch where tests are running against PHP 8. See https://travis-ci.com/github/matomo-org/matomo/jobs/475638482