Is this a good object oriented design for a file validator?
That isn't OOP, that's procedural, with a thin wrapping that looks like OOP.  OOP will require that your validation be done on objects that share data and behavior.

Depending upon your file, this may or may not make sense.

OTOH, good procedural code is definitely better than the common PHP "just run it all together inside a single page" that frequently gets used.

EDIT: Expanding my answer after a comment.

First off, I should be clear that I wasn't criticizing your existing code, just saying they don't make it OOP. OOP is about data and behavior not just having things called classes and constructors.

Secondly, EDI probably means transactions of different types with well definined behavior for the various types -- that would be an opportunity to use/benefit from OOP.  I would try to design a base class or interface that can be applied to all lines/transactions and then have derived classes for the header/footer and body lines.

You need to focus on the similaririty of your data and your process for handling/validating it, your OOP implemention will come out of that.  It's a little to early to be asking about OOP.