php字符串太長怎么辦

if (! function_exists('mbSubStr')){
 function mbSubStr($str, $length = 0, $append = true)
 {
 $str = trim($str);
 $strlength = strlen($str);
 if ($length == 0 || $length >= $strlength) {
 return $str; //截取長度等于0或大于等于本字符串的長度，返回字符串本身
 }elseif ($length < 0){ //如果截取長度為負數
 $length = $strlength + $length;//那么截取長度就等于字符串長度減去截取長度
 if ($length < 0) {
 $length = $strlength;//如果截取長度的絕對值大于字符串本身長度，則截取長度取字符串本身的長度
 }
 }
 if (function_exists('mb_substr')){
 $newstr = mb_substr($str, 0, $length, 'utf-8');
 }elseif (function_exists('iconv_substr')){
 $newstr = iconv_substr($str, 0, $length, 'utf-8');
 }else{
 //$newstr = trim_right(substr($str, 0, $length));
 $newstr = substr($str, 0, $length);
 }
 if ($append && $str != $newstr){
 $newstr .= '...';
 }
 return $newstr;
 }
}