Accessing multiple nested Arrays and PHPmailer variables

So I'm creating a file where users can add their content information for a frontend template. Whatever they type in this php file will show on the site. Sort of have two issues:

1. I need to be able to create a nested array and be able to call all or just one of the nested array elements.
2. How do I call a variable from the setup function to PHPmailer: $mail->Username and $mail->Password

Here is what I have that works so far:

function setup() {
    return array(
        'firstname' => "Your first name",
        'lastname' => "Your last name"
    );
}

extract (setup ());

Then I can call any element from the array into the HTML

<?php echo $firstname; ?>

Here's what I'd like to do:

function setup() {
    return array(
        'firstname' => "Your first name",
        'lastname' => "Your last name",
        /**
         * Would like a nested array but need to be able to call "nested" or any value within individually, anywhere in my html.
         */
        $nested = array(
            $set1 = array(
                "value1",
                "value2",
                "value3",
            ),
            $set2 = array(
                "value4",
                "value5",
                "value6",
            )
        ),
        /** 
         * I need the 'mail' and 'mailpass' to link to $mail->Username and $mail->Password
         * */ 
        'mail' => "example@example.com",
        'mailpass' => "password"
    );
}

extract (setup ());

Then call it, something like this:

<div>
  <h1><?php echo $nested[0][1]; ?></h1>
  <ul>
    <li><?php echo $nested[1][2]; ?></li>
    <li><?php echo $nested[1][2]; ?></li>
  </ul>
  <div>
    <?php echo $nested[$set1]; ?>
  </div>
  <div>
    <?php echo $nested[$set2]; ?>
  </div>
</div>

And have PHPmailer do this:

if ( array_key_exists( 'email', $_POST ) ) {
    //From this
    ..
    $mail->Username = 'example@example.com';
    // SMTP password
    $mail->Password = 'password';
    ..
    //To Something like this
    ..
    $mail->Username = $mail; // This is the variable from the setup function
    // SMTP password
    $mail->Password = $mailpass; // This is the variable from the setup function
    ..
}

Are loops required for these 2 issues? If so, how would I go about this? Thanks in advance

source https://stackoverflow.com/questions/68145342/accessing-multiple-nested-arrays-and-phpmailer-variables