Is my PHP code secure?

I'm using PHP and want to make an encryption method for storing a random value in the user's cookie as a method of authentication so they don't need to log back in every time they visit the site. Would the code below be secure?

$salt = "some random characters I made up";
hash('sha256', $salt.microtime(true).mt_rand(10000000,99999999));

Thanks!

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