source custom field

// to set a predefined input field for custom field from within functions.php
// 'Source' field below post editor
add_action('admin_menu', 'custom_meta_input');
add_action('save_post', 'save_custom_meta_input');
function custom_meta_input() {
    add_meta_box('custom_meta_input', 'Source', 'custom_meta_input_function', 'post', 'normal', 'high');
    // 'Source' is the label above the input area for this custom field
}
function custom_meta_input_function() {
    global $post;
    echo '<input type="hidden" name="custom_meta_input_hidden" id="custom_meta_input_hidden" value="'.wp_create_nonce('custom-meta-input-nonce').'" />';
    echo '<span>Input Source: </span><input style="width: 100%;" type="text" name="custom_meta_input_value" id="custom_meta_input_value" value="'.get_post_meta($post->ID,'_source_custom_meta_input_value',true).'" />';
    /* the custom field key is 'this_custom_meta_input';
    // i.e. whereever you need to use this custom filed in a template,
    // use <?php echo get_post_meta($post->ID,'_source_custom_meta_input',true); ?>
    */
}
function save_custom_meta_input($post_id) {
    if (!wp_verify_nonce($_POST['custom_meta_input_hidden'], 'custom-meta-input-nonce')) return $post_id;
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $post_id;
    $customMetaInputValue = $_POST['custom_meta_input_value'];
    update_post_meta($post_id, '_source_custom_meta_input_value', $customMetaInputValue);
}
////END to set a predefined custom field from within functions.php//