HTML input elements cannot hold more than one value. If you do need to encode an array into a form for some reason you’ll have to create an input element for each value as follows.
^ This, however. The OP’s input is hidden, so it would suggest to me they are looking for a way to move an array of variables from one place to another (script wise) along with the main form, for which I would probably suggest using a session to achieve.
Using session certainly achieves this within the PHP pages themselves. Since the OP hasn’t clarified what they need to do it’s hard to know what answer is best. But as session is such an obvious solution I posted with the assumption that PHP needs to share something with javascript. And who knows, the page might be building a form that will be submitted to a server not under the programmer’s control for some reason.