Calculate Time difference in php and setting the status

I want to get the current time of server and fetch the time present in database then based on that I want to calculate the time difference of both the time.Actually I want to show in my app that if time difference exceeds 10 minutes then status should changed to unavailable otherwise it should display available status??Plzz help me in this and provide code for this if possible..Moreover how can I convert a variable into string in PHP??

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