Nested namespaces are not allowed
-----
<?php
namespace A {
    namespace B {

    }
}
-----
Namespace declarations cannot be nested from 3:5 to 5:5
array(
    0: Stmt_Namespace(
        name: Name(
            parts: array(
                0: A
            )
        )
        stmts: array(
            0: Stmt_Namespace(
                name: Name(
                    parts: array(
                        0: B
                    )
                )
                stmts: array(
                )
            )
        )
    )
)