Since PHP 7.4 has a release candidate, it can be considered stable. Now we can start thinking about the compatibility.
The following is the problems from phpunit unit test:
* `ReflectionType::__toString()` is deprecated -- T192167 ([[ https://github.com/php/php-src/blob/php-7.4.0RC1/UPGRADING#L395-L398 | php-7.4.0RC1 note ]])
```
Function ReflectionType::__toString() is deprecated
tests/phpunit/unit/includes/watcheditem/NoWriteWatchedItemStoreUnitTest.php:225
tests/phpunit/MediaWikiUnitTestCase.php:110
maintenance/doMaintenance.php:99
```
* Cannot use "parent" when current class scope has no parent ([[ https://github.com/php/php-src/blob/php-7.4.0RC1/UPGRADING#L369-L371 | php-7.4.0RC1 note ]])
```
Cannot use "parent" when current class scope has no parent
vendor/wikimedia/remex-html/RemexHtml/TreeBuilder/ForeignAttributes.php:142
vendor/wikimedia/remex-html/RemexHtml/TreeBuilder/InBody.php:414
vendor/wikimedia/remex-html/RemexHtml/TreeBuilder/Dispatcher.php:385
vendor/wikimedia/remex-html/RemexHtml/Tokenizer/Tokenizer.php:580
vendor/wikimedia/remex-html/RemexHtml/Tokenizer/Tokenizer.php:359
vendor/wikimedia/remex-html/RemexHtml/Tokenizer/Tokenizer.php:168
includes/tidy/RemexDriver.php:66
tests/phpunit/unit/includes/tidy/RemexDriverTest.php:323
tests/phpunit/MediaWikiUnitTestCase.php:110
```