Description:
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It seems to be an ambiguity issue and a probable future problem.
You can create a class and define a constant with the same name and vice-versa.
Who is the class and who is the constant?

For future problems: currently, the magic __toString method works as magic instance method even if it is declared as public static ( ... and inherited in instances as public without static ... but this is another problem ... ) If one day PHP would like to support a public static __toString method this ambiguity will be a problem to understand if we are trying to get them, or the possible defined constant.

Reproduce code:
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class Test{}
define('Test', 123);

// defined('Test') or class_exists('Test') ?



define('Test2', 'Test2');
class Test2{}

get_class(new Test2) === Test2; // true
echo Test2; // Test2
$ref = Test2;
new $ref; // an instanceof Test2 ... 

is_string(Test2); // true

Expected result:
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A fatal error, because a constant should be a unique and immutable value with a name that could not be used as class one and a class should be unique as well (if we cannot use the same name for two different classes, how can we have two totally different meaning with a sngle name, the class and the constant)

Actual result:
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It is possible to create a class and then define a constant with the same name, and it is possible to define a constant and then a class with the same name.