Use of Preg_replace

Read the manual first ... Take a closer look

http://php.net/manual/zh/function.preg-replace.php

Are you a little confused about backwards references? Keep looking.

Http://deerchao.net/tutorials/regex/regex.htm main look back to quote that column

Do you understand me?

Yes! The grouping is determined by (). Left-to-right, with the left parenthesis of the group as a flag, the first occurrence of the group number is 1, the second is a 2

So you know the reason for ${1}?

Okay, that's all.

Use of Preg_replace