PHP method to intercept mixed Chinese and English strings

//截取想这样的字符串
a李三

/**
 * 
 * 中英混合的字符串截取
 * @param unknown_type $sourcestr
 * @param unknown_type $cutlength
 */
function assoc_substr($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
    } 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;
}