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

?>
<?php

$str = "a 1b 2