Why doesn't my echo and var_dump work in aws-sdk-php?

For debugging my amazon integrated site I have put echo command and var_dump functions in some of the source files of aws-sdk but nothing is printing.

I thought I might be looking at different source code but if I do any syntactical error (purposefully) in the code that will throw the error. Why is this? Is there something to do with composer? because I installed it using composer