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<?php
$json='{"clusterno":167145108,"clustertype":1,"subid":"0","newperson":"欢迎【.dazuo.昵称.dayou.】来到群【.dazuo.群名.dayou.】.enter.信息：.enter..dazuo.信息.dayou..enter..dazuo.退群次数.dayou..enter.请改群名片：帮派名┃职位 或者 帮派名┃职位┃昵称.enter.名称不符.douhao.不改名者送去夏威夷...enter..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..jhao..enter.\.douhao.\是机器人.douhao.请直接无视忽略.","removeperson":"","expdatetime":"2021-10-18 23:50:42","userid":"1","username":"admin","updatecode":"7ECEDB4E1037476EAAF5FB5E8340AAEE"}';
$json=addcslashes($json,'\\');
var_dump(json_decode(iconv('GBK','UTF-8',$json),true));
?>

------解决方案--------------------
utf-8编码文件就不需要再  iconv了.参考楼上的.

------解决方案--------------------
探讨


反斜杠转义，转换编码
PHP code
<?php
$json='{"clusterno":167145108,"clustertype":1,"subid":"0","newperson":"欢迎【.dazuo.昵称.dayou.】来到群【.dazuo.群名.dayou.】.enter.信息：.enter..dazuo.信息.dayou..enter..dazuo.退群次数.dayou..……