Function: mb_substr( $str, $start, $length, $encoding )
$str, a string that needs to be truncated
$start, the beginning of the truncation
$length, length (note that this is different from mb_strimwidth, 1 represents a Chinese character)
$encoding, encoding, I set it to utf-8
Example: Cut off the title of the article and control it to 15 texts
$str, a string that needs to be truncated
$start, the beginning of the truncation
$length, length (note that this is different from mb_strimwidth, 1 represents a Chinese character)
$encoding, encoding, I set it to utf-8
Example: Cut off the title of the article and control it to 15 texts