How to check file types of uploaded files in PHP?

Take a look at mime_content_type or Fileinfo. These are built-in PHP commands for determining the type of a file by looking at the contents of the file. Also check the comments on the above two pages, there are some other good suggestions.

Personally I've had good luck using something that's essentially system("file -bi $uploadedfile"), but I'm not sure if that's the best method.

IMHO, all MIME-type checking methods are useless.

Say you've got which should have MIME-type application/pdf. Standard methods are trying to find something that looks like a PDF header (%PDF- or smth. like that) and they will return 'Okay, seems like this is a PDF file' on success. But in fact this doesn't means anything. You can upload a file containing only %PDF-1.4 and it will pass MIME-check.

I mean if the file has an expected MIME-type - it will always pass the MIME-type check otherwise the result is undefined.