From what I can tell from the MIT license it means you can do absolutely anything with it so long as you keep the copyright notice in.
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
The original author also cannot be sued since it comes with no warranty.
If you use a JS library under MIT there are no problems if it's on a public web server. However, with PHP the code is not visible to all. This might be a silly question but to meet the license requirements do you simply need to keep the copyright message in on your PHP files—or do you need to have it somewhere publicly on your website? I'd guess the former but wasn't sure.
Finally, if you merged the source from several MIT licensed sources would you just need to have a copyright message left in for each source you used?
As long as the notice accompanies the code it came with, that's all you have to do.