<?php

class test
 {
 public function __set($k,$v)
 {
 echo '$k->'.$k.'<br>';
 echo '$v->'.$v.'<br>';
 }

public function __get($k)
 {
 echo '$k->'.$k.'<br>';
 }
 }

$b = new test();
 (new test())->a;

(new test())->a=1;

报错信息：Fatal error: Cannot use temporary expression in write context in D:\phpstudy\WWW\index.php on line 16

PHP环境 PHP 7.0.12-Nts

但是把以上代码稍微改一下，执行就正常了

<?php
class test
{
 public function __set($k,$v)
 {
 echo '$k->'.$k.'<br>'; 
 echo '$v->'.$v.'<br>'; 
 }
 public function __get($k)
 {
 echo '$k->'.$k.'<br>'; 
 }
}
$b = new test();
(new test())->a;
$b ->a=1;

执行正常 结果如下
$k->a
$k->a
$v->1