Callable type hint
-----
<?php

function a(callable $b) {}
-----
array(
    0: Stmt_Function(
        byRef: false
        params: array(
            0: Param(
                name: b
                default: null
                type: callable
                byRef: false
            )
        )
        stmts: array(
        )
        name: a
    )
)