In this PHP code, preg_replace function is used to perform a regular expression search and replace operation on the $data string. The regular expression pattern (\d)(?=(\d\d\d)+(?!\d)) matches a digit (\d) followed by every three digits ((\d\d\d)+), as long as it is not followed by another digit ((?!\d)). The matched digits are then replaced with the digit ($1) followed by a dot (.).
Note that in PHP, the forward slash / delimiters used in JavaScript regular expressions are not required. Additionally, the g flag to perform a global search is not needed in PHP, as preg_replace by default replaces all occurrences of the pattern.