Its fast, powerful, dynamic and able to connect to almost any DB on the planet. For instance, you say you have a lots of subpages (like a products page for a store (say 100,000 products), you could have 100,00 pages and maintain them all or
you can use PHP and a DB with some templates. Your 100,000 is reduced to 1 or 2 pages that call the DB and fill in the template. You can place your nav stuff in an include file and have to maintain only 1 nav file, 1 file for a DB connection string info, again only 1 page.
You can easily integrate new technologies like XML into PHP and make truly compliant sites with XML and CSS. You can write files on the fly (ie create a XML file from stuff in the D😎