Metabox

<?php

/* ############################################### */
// Kod w header.php
$meta = strb_get_meta('_stref_ban');
//echo "Post: ".$post_id."<br><br>";
//echo serialize( $get_meta );
echo 'Meta: ' . $meta . '<br>';
if ($meta == '1') {
   echo 'Tresc 1';
} elseif ($meta == '2') {
   echo 'Tresc 2';
}
/* ############################################### */
// Kod w functions.php
function strb_get_meta( $value ) {
    global $wp_query;
    $post_id = $wp_query->post->ID;

    $field = get_post_meta( $post_id, $value, true );
    if ( !empty( $field ) ) {
        return is_array( $field ) ? stripslashes_deep( $field ) : stripslashes( wp_kses_decode_entities( $field ) );
    } else {
        return false;
    }
}

function strb_add() {
    $screens = ['post', 'page'];
    foreach ($screens as $screen) {
        add_meta_box(
            'strb_box_id',          // Unique ID
            'Strefa banerowa',      // Box title
            'strb_html',            // Content callback, must be of type callable
            $screen                 // Post type
        );
    }
}
add_action('add_meta_boxes', 'strb_add');

function strb_html($post) {
    $value = get_post_meta($post->ID, '_stref_ban', true);
    ?>
    <select name="strb_field" id="strb_field" class="postbox">
        <option value="">Domyślna</option>
        <option value="1" <?php selected($value, '1'); ?>>Opcja 1</option>
        <option value="2" <?php selected($value, '2'); ?>>Opcja 2</option>
    </select>
    <?php
}

function strb_save_postdata($post_id) {
    if (array_key_exists('strb_field', $_POST)) {
        update_post_meta(
            $post_id,
            '_stref_ban',
            filter_input( INPUT_POST, 'strb_field', FILTER_VALIDATE_INT)
        );
    }
}
add_action('save_post', 'strb_save_postdata');
/* ############################################### */