html截取函数，目前只支持英文

function html_substr($s,$len) {
$a=preg_split(
[email='@(]*>)@',$s,-1,PREG_SPLIT_DELIM_CAPTURE|PREG_SPLIT_OFFSET_CAPTURE]'@(]*>)@',$s,-1,PREG_SPLIT_DELIM_CAPTURE|PREG_SPLIT_OFFSET_CAPTURE[/email]
);
$str=array();
$stack=array();
$single_tag=explode(' ','br hr img');
$r_len=0;
foreach ($a as $key=>$val) {
  if($key%2==0) {
   $now_len=strlen($val[0]);
   if($now_len+$r_len])*>@isU',$val[0],$ar]'@])*>@isU',$val[0],$ar[/email]
);
   if(!in_array($ar[2][0],$single_tag)) {
    if($ar[1][0]=='/') array_shift($stack);
    else array_unshift($stack,$ar[2][0]);
   }
   $str[]=$val[0];
  }
}
if(!empty($stack)) {
  foreach ($stack as $v) {
   $str[]='';
  }
}
return join('',$str);
}