improved uniqueness of bucket name and consistency of output statements

* Modified the call to `uniqid()` to use the [`more_entropy` flag](http://php.net/uniqid#refsect1-function.uniqid-parameters). This should help increase the uniqueness of the bucket name for those running the sample.
* Replaced all calls to `printf()` with `echo` using string interpolation. This is generally more familiar and improves consistency of the sample code by making all of the output statements in the sample use `echo` exclusively.