Assigning WP functions which echo to PHP variables

Question

I am creating a settings page. If I use the default example it works fine.

<form method="post" action="options.php">
    <?php if(isset( $_GET['settings-updated'])) { ?>
    <div class="updated">
        <p>Settings updated successfully</p>
    </div>
    <?php } ?>

    <textarea name="setting_1"><?php echo get_option('setting_1'); ?></textarea>

    <?php 

    settings_fields('my_settings');
    do_settings_sections('my_settings');

    submit_button(); 

    ?>
</form>

However, I am using PHP to more easily manipulate the HTML (start of complex functionality) so I changed submit_button() to get_submit_button() in order to assign the returned value to a variable.

$output .= '</div>'
$output .= get_submit_button();
$output .= '</form>';

This works as expected. However, I cannot seem to find a similar function for settings_fields('my_settings') and do_settings_sections('my_settings').

These functions echo the output rather than return the output. This means these functions echo their output outside of the form I am constructing with PHP.

How is it possible to capture echoed output from such functions and assign it to a variable in PHP?

Answer 1

Found it, I think this was suggested to me in another answer.

$output .= ...

ob_start();
settings_fields('my_settings');
do_settings_sections('my_settings');
$output .= ob_get_clean();