I am able to connect with my ad manager account using Google Ad Manager API (https://github.com/googleads/googleads-php-lib/tree/master/examples/AdManager). I am getting data i.e all companies, all placement on my command line. But confused on getting impressions.
How to get an impression of a placement?
First, you will need a query. Using the UI is the easiest way to build a query, and see the results. Here's the guide from Google to create a query on the UI.
Then, you will save the query and follow the ReportService/RunSavedQuery.php example to download the impression report in your program.
Please note that the examples are removed every 4 quarters and replaced by newer examples. So, if the example link above doesn't work, the example might have been replaced with a newer one.
This question is not a PHP specific question. Indeed, it is a "how to use the product" question, and is best suitable to be answered on the Google Ad Manager product forum. If you raise a question on the forum, the support staff may get back to you on a secured channel to help you build the query.