There are others who are real tech guys and could answer better I'm sure, but since no one else has answered yet here's my two cents. I do know a little about it.
php is the most widely used open source (translation, "free") programming language for web work and I think the most widely used, period. It is used often with MySQL (a database program) which is also open source . ASP does similar stuff, as do many other programs, but they cost more. ASP is sometimes labeled M$ ASP. Usually the line, "You get what you pay for" is true, but in this case many programmers would say that php is the best even if it is free.
If you ever expect to use contact forms to collect data or anything like that it is a primary way to go. Also, a lot of sites use php files because you can put programming on your domain site that can't be seen in the source page for your site. It is one of the easiest languages to learn but is very powerful.
A simple web project might not need it, but if you anticipate any potentially more complicated projects in the future you might want to plan for it now. Personally I would recommend it. It was one of the essential requirements I looked for in choosing my web host.