Undefined constant 'PHP_SESSION_ACTIVE'

Im trying to run Grav on a FreeBSD 11.2 server with Apache 2.4.41 and PHP 7.2.21.
It runs a bunch of other sites such as a few Wordpress and Mattermost installation without problems, so the server seems to work.
I’ve tried downloading the latest zipfile and made several pulls from git, but I always just get the error “Undefined constant ‘PHP_SESSION_ACTIVE’”.
I even tried running the built-in PHP webserver, but get exactly the same error.

The latest line from the log is:
[2019-10-16 21:40:17] grav.CRITICAL: Undefined constant ‘PHP_SESSION_ACTIVE’ - Trace: #0 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/Session/Session.php(58): Grav\Framework\Session\Session->isSessionStarted() #1 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Service/SessionServiceProvider.php(93): Grav\Framework\Session\Session->__construct(Array) #2 /usr/local/www/miss.bugs.se/grav/vendor/pimple/pimple/src/Pimple/Container.php(118): Grav\Common\Service\SessionServiceProvider->Grav\Common\Service{closure}(Object(Grav\Common\Grav)) #3 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Processors/InitializeProcessor.php(52): Pimple\Container->offsetGet(‘session’) #4 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\InitializeProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #5 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #6 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Processors/DebuggerProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #7 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\DebuggerProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #8 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #9 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Processors/ErrorsProcessor.php(27): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #10 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ErrorsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #11 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #12 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Processors/LoggerProcessor.php(48): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #13 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\LoggerProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #14 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #15 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Processors/ConfigurationProcessor.php(28): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #16 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(45): Grav\Common\Processors\ConfigurationProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler)) #17 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php(57): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #18 /usr/local/www/miss.bugs.se/grav/system/src/Grav/Common/Grav.php(249): Grav\Framework\RequestHandler\RequestHandler->handle(Object(Nyholm\Psr7\ServerRequest)) #19 /usr/local/www/miss.bugs.se/grav/index.php(53): Grav\Common\Grav->process() #20 {main} [] []

Any ideas?