7.4+ compatibility check

[dereuromark]

There are 3 important RFCs that was introduced in PHP 7.4:

• https://wiki.php.net/rfc/arrow_functions_v2
• https://wiki.php.net/rfc/null_coalesce_equal_operator
• https://wiki.php.net/rfc/typed_properties_v2

We should check that those don't break anything.

[dereuromark]

Specifically, the following issues seem to be not covered

$fn1 = fn($x)=>$x + $y;

should be

$fn1 = fn ($x) => $x + $y;