Having coding style conventions is a good idea, because it helps developers not get distracted by code written in a different style when working on code they did not write. It will make your code base superficially cleaner. It's great if you can automate it, but there is usually no need to go through great lengths to comply (unless the current style is terrible). If you already have a good-enough standard, stick to it.
Code smell is something different though: it is (a set of) symptoms that may indicate a deeper problem with the code. Examples are cyclomatic complexity, long method names, large classes, undescriptive names, duplicate code, etc. This is usually much more problematic, as it may thoroughly hurt the maintainability of your code. You should definitely solve these problems.
PHP CodeSniffer appears to be mainly developed for checking style conventions, not code smell. If you can use it to help enforce style conventions, great. But beware that it will not make your code base substantially better. You will want to do manual reviews to accomplish that.
If you want to use it to check if you comply to your current standard, that appears to be possible, see the answer to the question "I don't agree with your coding standards! Can I make PHP_CodeSniffer enforce my own?" in their FAQ.
