php - Why can I use a string literal as a class in php7?

Consider the following code:

class foo {
    static $bar = 'baz';
}
var_dump('foo'::$bar);

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It throws an error in PHP5 (as expected):

Parse error: syntax error, unexpected '::' (T_PAAMAYIM_NEKUDOTAYIM) in [...][...] on line 4

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But it works without an issue in PHP7 and outputs:

string(3) "baz"

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Is that intentional or a bug?

Answer

I think it's because they rewrote stuff regarding evaluation.

like the following is not possible in PHP5 but in PHP 7:

echo (new X)->toString();

Same would go for

echo ('X')::$bar

See Changes to the handling of indirect variables, properties, and methods

This is mainly about the left-to-right evaluation but it also affects the evaluation in general.

More information can be found on PHP RFC: Uniform Variable Syntax (Status: implemented) - Thanks to Nikic:

This RFC proposes the introduction of an internally consistent and
complete variable syntax. To achieve this goal the semantics of some
rarely used variable-variable constructions need to be changed.