Arrow function
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<?php

fn($a) => $a;
fn($x = 42) => $x;
fn(&$x) => $x;
fn&($x) => $x;
static fn($x, ...$rest) => $rest;
fn(): int => $x;
-----
!!php7
fn($a) => $a;
fn($x = 42) => $x;
fn(&$x) => $x;
fn&($x) => $x;
static fn($x, ...$rest) => $rest;
fn(): int => $x;