Variable syntaxes
-----
<?php

$a;
${'a'};
${foo()};
$$a;
$$$a;
$$a['b'];
-----
array(
    0: Expr_Variable(
        name: a
    )
    1: Expr_Variable(
        name: Scalar_String(
            value: a
        )
    )
    2: Expr_Variable(
        name: Expr_FuncCall(
            name: Name(
                parts: array(
                    0: foo
                )
            )
            args: array(
            )
        )
    )
    3: Expr_Variable(
        name: Expr_Variable(
            name: a
        )
    )
    4: Expr_Variable(
        name: Expr_Variable(
            name: Expr_Variable(
                name: a
            )
        )
    )
    5: Expr_Variable(
        name: Expr_ArrayDimFetch(
            var: Expr_Variable(
                name: a
            )
            dim: Scalar_String(
                value: b
            )
        )
    )
)