PHP：静态属性中的类实例化会引发异常

I'd like to create a class (e.g. Bar) which has a private static property. This property should be an array of objects of Foo.

<?php

class Foo {

}

class Bar {
    private static $classes = array(new Foo(), new Foo());

    public static function testClasses() {
        var_dump(self:$classes);
    }
}

Bar::testClasses();

However this code throws an exception:

PHP Parse error: syntax error, unexpected 'new' (T_NEW), expecting ')' in [...]/test.php on line 8

Can somebody explain me why this is not possible?

From the docs:

This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

Your use of new to instantiate the classes in the property definition Is dependent on run-time information