PHP English string interception code (to ensure word integrity)

/**
 * Complete word interception
 * Edited and organized by bbs.it-home.org
 * @param $str
 * @param $start
 * @param $length
 *
 * @return string
*/
 public static function usubstr($str, $start, $length = null)
 {

 // Intercept normally first.
 $res = substr($str, $start, $length);
 $strlen = strlen($str);

 /* Then determine whether the first and last 6 bytes are complete (not incomplete) */
 // If the parameter start is a positive number
 if ($start >= 0) {
 //Truncate about 6 bytes forward
 $next_start = $start + $length; //Initial position
 $next_len = $next_start + 6 <= $strlen ? 6: $strlen - $next_start;
 $next_segm = substr($str, $next_start, $next_len);
 // If the first byte is not the first byte of the complete character, then intercept about 6 bytes
 $prev_start = $start - 6 > 0 ? $start - 6 : 0;
 $prev_segm = substr($str, $prev_start, $start - $prev_start);
 } // start is a negative number
 else {
 // Cut about 6 bytes forward
 $next_start = $strlen + $start + $length; // Initial position
 $next_len = $next_start + 6 <= $strlen ? 6 : $strlen - $next_start;
 $next_segm = substr($str, $next_start, $next_len);

 // If the first byte is not the first byte of the complete character, then intercept about 6 bytes later.
 $start = $strlen + $start;
 $prev_start = $start - 6 > 0 ? $start - 6 : 0;
 $prev_segm = substr($str, $prev_start, $start - $prev_start);
 }
 // Determine whether the first 6 bytes match utf8 rules
 if (preg_match('@^([x80-xBF]{0,5})[xC0-xFD]?@', $next_segm, $bytes)) {
 if (!empty($bytes[1] )) {
 $bytes = $bytes[1];
 $res .= $bytes;
 }
 }
 // Determine whether the last 6 bytes comply with utf8 rules
 $ord0 = ord($res[0]);
 if (128 <= $ord0 && 191 >= $ord0) {
 // Intercept from the back and add it in front of res.
 if (preg_match('@[xC0-xFD][x80-xBF]{ 0,5}$@', $prev_segm, $bytes)) {
 if (!empty($bytes[0])) {
 $bytes = $bytes[0];
 $res = $bytes . $res;
 }
 }
 }
 if (strlen($res) < $strlen) {
 $res = $res . '...';
 }
 return $res;
 }