Please check my method to autheniticate mobile app with PHP server
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I'm building an android app that needs to authenticate with a php service. My current approach is when the user initially logs in, a unique id is generated using uniqid(). This uid is then stored in a table on the database along with their username.The user is then passed back this uid. 

Subsequently all further requests to the server will send the the uid and their username, which will be then checked in the database to authenticate the user.

Is this approach OK? or are their massive security holes that I am missing?