How to construct a string based on regex and related parameters in PHP?

You propose that if you pass the regular expression (?P<id>\d*)(/(?P<title>.*))? to a function along with the parameters id=4 and title='' that the function would return 4. And that this would work for any regular expression. That is simply impossible. Your regular expression is an example that such a function could never support.

If you call preg_match with your regular expression on the string 4 then the match results will return 4 for the capturing group id and the empty string for the capturing group title. If you call preg_match with the same regex on the string 4/ then the match results will also be 4 for the capturing group id and the empty string for the capturing group title. PHP does not differentiate between capturing groups that match the empty string and capturing groups that do not participate in the regex at all. Both return the empty string. Notice that in your regex you did not only make the group optional with the question mark, the .* inside your capturing group is also optional.

Thus, we have two possible matches 4 and 4/ for which preg_match returns 4 for the capturing group id and the empty string for the capturing group title. So how is your requested reverse function supposed to determine whether it should return 4 or 4/ for your two capturing groups? It can't be done without additional information.

In fact, do you realize your regex also matches / as well as the empty string? Everything in your regular expression is optional!

how to get the original matched string if we have id and title already?

Original Matched string should be in

$0

The whole regex match is always located in the group 0, so you can get this match in the matches array of preg_match() by accessing index 0 an/or using $0 or \0 in the replacement of preg_replace()

Not sure I understand the question, you mean constructing the string like this?

$string = $id;
if ( isset($title) ) {
    $string .= "/$title";
}