Authenciating users between Android and PHP Webservice

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Mobile

PHP

Android

webservice

Im creating a simple android application,it requires some details to stored and retived from a web-service coded in php.I have planned to create PHP script like www.mycoolservice.com/domagic.php Android App will request the script some details via HTTP POST and the script will send the data using json encoding.

My question is about authentication and security

Which is the best way to authenticate a user with the server,should i send the username and password with every HTTP POST(encrypted)? Also if there are large number of users using the same script at a time,will there be some concurrency and efficiency issues?
Im just a beginner in webservices,Please guide me.