Create meta-box to fetch all the cities from the database

functions.php

<?php
/**
* Register meta box(es).
*/
add_action( 'add_meta_boxes', 'city_register_meta_boxes' );
function city_register_meta_boxes() {
$post_type = get_post_type();
add_meta_box( 'city', 'City', 'city_display_callback', $post_type, "side", "low", 'default');
}
function city_display_callback($post){
wp_nonce_field(basename(__FILE__), "meta-box-nonce");
?>
<div style="overflow: auto;width: 100%; height: 200px;">  
  <!--- -->
  <?php 
global $post,$wpdb;
$countries = $wpdb->get_results( "SELECT * FROM wp_city" ); ?>
  <ul id="ads_catchecklist">
    <?php
$cities = get_post_meta($post->ID,"city", true);
$cities = explode(',',$cities);
foreach($countries as $country) 
{
?>
    <li style="list-style-type: none;" id="ads_city_<?php echo $city->id; ?>" class="popular-city">
      <label class="selectit">
        <input name="city[]" type="checkbox" value="<?php echo $country->cityname;?>" 
               <?php if (in_array( $country->cityname, $cities)){ echo 'checked'; } ?> >
        <?php echo $country->cityname; ?>
      </label>
    </li>
    <?php
}
?>
    </div>
  </ul>
<?php  
}
function save_global_notice_meta_box_data( $post_id ) {
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) { return; }
if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {
if ( ! current_user_can( 'edit_page', $post_id ) ) {
return;
}
}
else {
if ( ! current_user_can( 'edit_post', $post_id ) ) { return; }
}
$my_data =  $_POST['city'];
$my_data  = implode(',',$my_data);
update_post_meta( $post_id, 'city', $my_data );
}
add_action( 'save_post', 'save_global_notice_meta_box_data' );