<?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

?>