判断字符串是否为 utf-8 编码

<?php
/**
 * 判断字符串是否为utf8编码，英文和半角字符返回ture
 * @author ruxing.li
 * @param $string
 * @return bool
 */
function is_utf8($string) {
    return preg_match('%^(?:
                    [\x09\x0A\x0D\x20-\x7E] # ASCII
                    | [\xC2-\xDF][\x80-\xBF] # non-overlong 2-byte
                    | \xE0[\xA0-\xBF][\x80-\xBF] # excluding overlongs
                    | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2} # straight 3-byte
                    | \xED[\x80-\x9F][\x80-\xBF] # excluding surrogates
                    | \xF0[\x90-\xBF][\x80-\xBF]{2} # planes 1-3
                    | [\xF1-\xF3][\x80-\xBF]{3} # planes 4-15
                    | \xF4[\x80-\x8F][\x80-\xBF]{2} # plane 16
                    )*$%xs', $string);
}