Since PHP 7.4 has a release candidate, it can be considered nearly stable. Now4.0 has been released, 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
```
[x] 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 ]])
[x] Deprecated: Array and string offset access syntax with curly braces is deprecated in `avro-php` lib. ([[ https://github.com/php/php-src/blob/php-7.4.0RC1/UPGRADING#L360-L362 | php-7.4.0RC1 note ]])
[ ] `MWExceptionHandlerTest::testGetRedactedTrace`: The stacktrace must have a function having an object has parameter; Failed asserting that false is true.