How can I extract the name of a $_POST variable, not its value_

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I am building a function that will validate the values in a form.How can I extract the actual name in string form from a $_POST variable?Ex: $_POST['first_name'] = 'Joe';I now want to grab the string 'first_name' and use it in a function to declare a variable by that name. I know I can use a foreach loop but isn't there an easier way to do this?