The header sent by Postman is not received in the PHP script
I don’t think that you can use an underscore in the header name of a custom header as it’s a feature that’s disabled by default. More information can be found here
You could test this out by removing this from the header name.
Seems to be old thread;I had same issue with new version of PostMan; Old version works fine (with USER and PW parameters).
NOT A RESOLUTION AN ALTERNATIVE TO MOVE ON (before it is resolved at PostMan's end):An Odd and Quick solutions is to add a new variable in Header (e.g. username and password: which will be sent as HTTP_USERNAME and HTTP_PASSWORD)...else you have to configure the .htaccess file, check PHP ini and WebServer (Apache/IIS)