php字符串太长怎么办

if (! function_exists('mbSubStr')){
 function mbSubStr($str, $length = 0, $append = true)
 {
 $str = trim($str);
 $strlength = strlen($str);
 if ($length == 0 || $length >= $strlength) {
 return $str; //截取长度等于0或大于等于本字符串的长度，返回字符串本身
 }elseif ($length < 0){ //如果截取长度为负数
 $length = $strlength + $length;//那么截取长度就等于字符串长度减去截取长度
 if ($length < 0) {
 $length = $strlength;//如果截取长度的绝对值大于字符串本身长度，则截取长度取字符串本身的长度
 }
 }
 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;
 }
}