Fix xdebug coverage undefined constant

Author: jbouzekri

.travis.yml

@@ -12,7 +12,7 @@ before_script:
   - if [ $TRAVIS_PHP_VERSION = "7.4" ]; then wget -O phpstan.phar https://github.com/phpstan/phpstan/releases/download/0.12.18/phpstan.phar ; fi
 
 script:
-  - php phpunit.phar --configuration ./build/travis-ci.phpunit.xml
+  - XDEBUG_MODE=coverage php phpunit.phar --configuration ./build/travis-ci.phpunit.xml
   - if [ $TRAVIS_PHP_VERSION = "7.4" ]; then php phpstan.phar analyse --no-progress -c phpstan.neon; fi
 
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