http://hhvm.com/blog/2020/01/13/hhvm-4.40.html

普通更新

HHVM 4.40 is released! This release marks the end of support for 4.33;
hhvm 4.34–4.39 remain supported, as do the 4.8 and 4.32 LTS releases.Breaking Changes specifying the same XHP attribute twice () is now
a syntax errorFuture ChangesA change last year unintentionally made the parser accept arbitrary types
for XHP children declarations, however: this remains unsupported by the XHP class library.
the typechecker does not validate children.
this can lead to impossible-to-satisfy children declarations, such as
references to undefined types.We are changing the typechecker to verify that XHP children declarations do
not refer to undefined types; in the context of XHP children declarations, the
following types will be considered valid by the typechecker: the case-sensitive special values pcdata, any, or empty.
XHP category names, such as %foo.
a defined, non-aliased, non-generic Hack type, such as a class name, XHP
classname, or interface.We expect this change to be included in 4.41.There are some additional things to be aware of: only the declaration is validated; the typechecker will not verify that
provided children actually match the declaration. This will remain a
runtime-only check, which must be supported by the XHP class library.
there are no typechecker checks for XHP categories in children declarations.
types other than XHP class names - such as standard classes or interfaces -
may not be fully supported by the XHP class library.
type aliases are not permitted.Additionally, this release includes disabled support for xhp class foo {}
as an alternative syntax to class :foo {}; this is to unblock future work on
improving interactions between XHP and namespaces - usage is not recommended at
this time.

HHVM (HipHop Virtual Machine)会将PHP代码转换成高级别的字节码（通常称为中间语言）。然后在运行时通过即时（JIT）编译器将这些字节码转换为x64的机器码

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