Creating Meta Box in Page/Post

add_action('admin_init', 'add_my_metabox');
function add_my_metabox() {
    add_meta_box('port_cat_id', 'Portfolio Categories', 'my_metabox_callback', 'page', 'side');   // adding metabox in Pages
    add_meta_box('port_cat_id', 'Portfolio Categories', 'my_metabox_callback', 'post', 'side');   // adding metabox in Posts
}
 
//function to add dropdown of Portfolio Categories
function my_metabox_callback() {
	  $getcat = get_categories(); 
	  $postmeta = get_post_meta($_GET['post'], 'key1', true);?>
      <select name="category_name">
      <option value="select category">Select Portfolio Category</option>
	 foreach($getcat as $portcat) { 
	    <option <?php echo ($postmeta == $portcat->term_id) ? "selected=selected" : ""?> value="<?php echo $portcat->term_id;?>"><?php echo $portcat->cat_name;?></option>	
	   }
     </select> 
}

//save added metabox content
add_action('save_post', 'update_page_portfolio_category');
function update_page_portfolio_category($post_id) {
    $mydata = $_POST['category_name'];
	update_post_meta($post_id, 'key1', $mydata);
}