奇怪的json_decode有关问题，为什么不能decode呢

奇怪的json_decode问题，，为什么不能decode呢？

PHP code

<?php    $rm = '5e?{"return":"error","errorcode":400,"errorinfo":"\u7528\u6237\u540d\u5df2\u88ab\u6ce8\u518c"}0'; //注意这里面有2个换行符，在5e后面和0前面    $a_rm = explode("\r\n", $rm);    $a_rm1 = $a_rm[1];    echo $a_rm1;    echo "<pre>";var_dump(json_decode($a_rm1, true));?>

PHP code

{"return":"error","errorcode":400,"errorinfo":"\u7528\u6237\u540d\u5df2\u88ab\u6ce8\u518c"}NULL

PHP code

$aa = '{"return":"error","errorcode":400,"errorinfo":"\u7528\u6237\u540d\u5df2\u88ab\u6ce8\u518c"}';    var_dump(json_decode($aa))