Variable syntaxes
-----
<?php

$a;
${'a'};
${foo()};
$$a;
$$$a;
$$a['b'];
-----
!!php5
array(
    0: Stmt_Expression(
        expr: Expr_Variable(
            name: a
        )
    )
    1: Stmt_Expression(
        expr: Expr_Variable(
            name: Scalar_String(
                value: a
            )
        )
    )
    2: Stmt_Expression(
        expr: Expr_Variable(
            name: Expr_FuncCall(
                name: Name(
                    parts: array(
                        0: foo
                    )
                )
                args: array(
                )
            )
        )
    )
    3: Stmt_Expression(
        expr: Expr_Variable(
            name: Expr_Variable(
                name: a
            )
        )
    )
    4: Stmt_Expression(
        expr: Expr_Variable(
            name: Expr_Variable(
                name: Expr_Variable(
                    name: a
                )
            )
        )
    )
    5: Stmt_Expression(
        expr: Expr_Variable(
            name: Expr_ArrayDimFetch(
                var: Expr_Variable(
                    name: a
                )
                dim: Scalar_String(
                    value: b
                )
            )
        )
    )
)