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<?php
function add_embed_tweet_meta_box() {
    add_meta_box(
        'paulund_embed_tweet_meta_box', // $id
        'Post Embed Tweets Meta Box', // $title
        'show_embed_tweet_meta_box', // $callback
        'post', // $page
        'normal', // $context
        'high'); // $priority
}
add_action('add_meta_boxes', 'add_embed_tweet_meta_box');

<?php
add_meta_box( $id, $title, $callback, $post_type, $context, $priority, $callback_args );

<?php 
    $meta_values = get_post_meta($post_id, $key, $single); 
?>

<?php
    $twitter_embed = get_post_meta($post->ID, "twitter_embed", true);
	
    if($twitter_embed != ""){
		?>
            What's being said on Twitter
        
        <?php
			echo $twitter_embed;
		?>
        
        <script src="//platform.twitter.com/widgets.js" charset="utf-8"></script><?php
	}
?>

<?php
/**************************************************************************
 * Save the meta fields on save of the post
 **************************************************************************/
function save_embed_tweet_meta($post_id) {   
    // verify nonce
    if (!wp_verify_nonce($_POST['custom_meta_box_nonce'], basename(__FILE__)))
        return $post_id;
        
    // check autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
        return $post_id;
        
    // check permissions
    if ('page' == $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id))
            return $post_id;
        } elseif (!current_user_can('edit_post', $post_id)) {
            return $post_id;
    }  
    
    $old = get_post_meta($post_id, "twitter_embed", true);
    
    $new = $_POST["twitter_embed"];
    
    if ($new && $new != $old) {
        update_post_meta($post_id, "twitter_embed", $new);
    } elseif ('' == $new && $old) {
        delete_post_meta($post_id, "twitter_embed", $old);
    }
}

add_action('save_post', 'save_embed_tweet_meta');

<?php

function show_embed_tweet_meta_box() {
    global $post;  
        $meta = get_post_meta($post->ID, 'twitter_embed', true);  
	
    // Use nonce for verification  
	echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';  
   
   echo '<table class="form-table">';   
        // begin a table row with
        
        echo '<tr>
            <th><label for="twitter_embed">Twitter Embed</label></th>
            <td><textarea name="twitter_embed" id="twitter_embed" cols="60" rows="4">'.$meta.'</textarea>
		        <span class="description">Use to embed tweets on your post.</span></td>
            </tr>';
            
    echo '</table>';
}

<?php
if (!wp_verify_nonce($_POST['custom_meta_box_nonce'], basename(__FILE__)))
        return false;