Forum project using OOP PHP
back: I'm not sure what this function is used for (and the name doesn't seem that great), but it is open to XSS. An attacker could create a page at http://evil.com/?&lt;img src=onerror=alert(1)&gt;, which would redirect the user to your back page, which would then echo the XSS payload. 
you should always die after a header redirect (clients do not have to follow it, so any code after the redirect may be executed, which may have security implications).
your indentation is inconsistent, and 1 or 2 spaces aren't really enough. I would use 4 at a minimum to ensure readability.
your spacing is also off, and your positioning of curly brackets isn't consistent. You can easily solve all these formatting issues with any IDE.
try to always write out your variable names. Right now, fname may be clear to you (in context), but firstName is much easier to read.
a nested if-else in an if is often not so nice to read (especially with your formatting). It's difficult to see what happens when rowCount is not greater than 0. You could just combine the two ifs.
getColumn doesn't actually get anything, it echoes something. You could rename it to showColumn, but really, it should return an array of columns, which is then echoed in a view class.
the same goes for all the exceptions. Just catching them and echoing the message isn't a great way to handle it. If you can't recover in the function, just throw them upwards, and let the calling code decide what to do.