中英数字混合的字符串截取

/**
     * 中英数字混合的字符串截取
     * Param: 参数说明
     * 【in】$sourcestr string  截取字符串
     * 【in】$cutlength int  截取长度
     */
    public function assoc_substr($sourcestr, $cutlength) {
        $returnstr = '';
        $i = 0;
        $n = 0;
        $str_length = strlen ( $sourcestr ); //字符串的字节数
        while ( ($n < $cutlength) and ($i <= $str_length) ) {
            $temp_str = substr ( $sourcestr, $i, 1 );
            $ascnum = Ord ( $temp_str ); //得到字符串中第$i位字符的ascii码
            if ($ascnum >= 224) {//如果ASCII位高与224，
                $returnstr = $returnstr . substr ( $sourcestr, $i, 3 ); //根据UTF-8编码规范，将3个连续的字符计为单个字符
                $i = $i + 3; //实际Byte计为3
                $n ++; //字串长度计1
            } elseif ($ascnum >= 192){ //如果ASCII位高与192，
                $returnstr = $returnstr . substr ( $sourcestr, $i, 2 ); //根据UTF-8编码规范，将2个连续的字符计为单个字符
                $i = $i + 2; //实际Byte计为2
                $n ++; //字串长度计1
            } elseif ($ascnum >= 65 && $ascnum <= 90) {//如果是大写字母，
                $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
                $i = $i + 1; //实际的Byte数仍计1个
                $n ++; //但考虑整体美观，大写字母计成一个高位字符
            }elseif ($ascnum >= 97 && $ascnum <= 122) {
                $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
                $i = $i + 1; //实际的Byte数仍计1个
                $n ++; //但考虑整体美观，大写字母计成一个高位字符
            }elseif($ascnum>=48 && $ascnum<=57){
                $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
                $n ++; //但考虑整体美观，大写字母计成一个高位字符
            }else {//其他情况下，半角标点符号，
                $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
                $i = $i + 1;
                $n = $n + 0.5;
            }
        }
        return $returnstr;
    }