It's not "PHP" - that's the language being used. XAMPP usually uses Apache for it's web server, and it's the web server that determines the port.
Look for http.conf. Edit it, and change the port to "80". Here are more details:
How to change XAMPP apache server port?
I believe you might also be able to use "xampp.ini":
Busy... Apache started [Port 80]
===============================================
ADDENDUM:
I don't thnk you understand.
There's ALWAYS a "port".
If you don't specify the port, it defaults to "80" (for http) or "443" (for https).
EXAMPLES:
http://localhost:8080/index.php: you're explicitly saying "index.php" and "port 8080".
http://localhost/index.php: port 80 is implicit.
http://localhost:80/index.php: port 80 is explicit. Examples 2) and 3) are EQUIVALENT. And finally,
http://localhost implies a) port 80, and b) index-dot-something: index.html, index.htm, index.php, index.jsp, etc. - whatever you've configured as a "default page".
'Hope that helps...