PHP实现过滤掉非汉字字符只保留中文字符

<?php

$str = "a 1b 2b＜中文＞。xxyy字符";

//转换 gb2312 -> utf-8
$str = mb_convert_encoding($str, 'utf-8', 'gb2312');

preg_match_all('/[x{4e00}-x{9fff}]+/u', $str, $matches);
$str = join('', $matches[0]);

//转换 utf-8 -> gb2312
$str = mb_convert_encoding($str, 'gb2312', 'utf-8'); 

echo $str; //输出 中文字符

?>