I am using a library that is under the MIT license. I never used a library that I needed to include the license before, so I have no idea where to add them. I read that I should add them in my file, but the thing is my project is a site constituting of php files, is there a certain file that I need to add the license to or all the files? Also what should I add? The project came with a LICENSE.txt file and contains text. The project I am using is jsModal.
I am not a lawyer, but here are my recommendations.
The main requirement of the MIT license can be found on the jsModal license page
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
If your project is a website, you should state that you are using jsModal that it is licensed under the MIT license. You need to make the entire text of the jsModal license page (including the copyright information) available directly on your site. I do NOT think a simple hyperlink the the jsModal page is sufficient. The text must be a part of your web site. If the web site is the only place you are using jsModal, you do not need to add any licensing information to any other files. A single page stating you are using jsModal with the entire text of the MIT license is sufficient.
Edit: You should make sure that this license/copyright information is visible to the end user. Typical users won't look at your code comments, so code comments by themselves are not sufficient. You only need a single page for this license & copyright information. You don't need to add it to every page of your website.