WordPress Plugin: Authors may only assign existing post_tags not add new

ppp-posttags-select-only.php

<?php
/*
 * Plugin Name: PPP Posttags Select Only
 * Plugin URI: http://gby.at
 * Description: Authors may only assign existing post_tags not add new
 * Author: 	Gabriele Laesser
 * Author URI:  http://www.xisign.com
 * License:     GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html 
 * Version: 	1.0
 * Text Domain: ppp-posttags-select-only
 * Domain Path: /languages
 */
 
	function pppf_post_tags_select_init() {
		load_plugin_textdomain( 'ppp-posttags-select-only', false, dirname( plugin_basename( __FILE__ ) ) . '/languages/' );
	}
	add_action('plugins_loaded', 'pppf_post_tags_select_init');
 
 
	function pppf_remove_tagsdiv() {
		if ( current_user_can('edit_pages') ) return;
		remove_meta_box( 'tagsdiv-post_tag', 'post', 'side' ); 
	} 
	add_action( 'admin_menu', 'pppf_remove_tagsdiv' );

	/**
	 * Retrieve the custom metabox post_tags array
	 * must be the slugs otherwise wordpress will set new post_tags from ids
	 *
	 */	
	function pppf_get_the_tags() {
		
		$terms = get_terms( array(
			'taxonomy' 	 => 'post_tag',
			'hide_empty' => false,
		) );
		
		if ( !$terms ) return array();
		
		foreach ( $terms as $term ) {
			$post_tags[$term->slug] = $term->name;
		}
		
		return $post_tags;
		
	}
	
	/**
	 * Retrieve all post_tags set in post-ID
	 * must be the slugs
	 *
	 */
	function pppf_get_post_tags( $post_id ) {

		$tags = wp_get_post_tags( $post_id );
		
		if ( !count( $tags ) ) {
			return array();
		}
		
		foreach ( $tags as $tag ) {
			$post_tags[] = $tag->slug;
		}
		
		return $post_tags;
	}	
	
	/**
	 * the metabox to select post_tags for a post
	 *
	 */
	function pppf_custom_tag_meta_box( $post, $box ) {
		if ( current_user_can('edit_pages') ) return;
		$post_tags 		= pppf_get_the_tags();
		$selected_tags  = pppf_get_post_tags( $post->ID );
		?>
		<div id="custom-post_tags-select">
			<fieldset>
				<legend class="screen-reader-text"><?php esc_html_e( 'Tags', 'ppp-posttags-select-only' ); ?></legend>
				<?php foreach ( $post_tags as $tag => $label ) : ?>
				<br /><input type="checkbox" name="tax_input[post_tag][]" id="post_tag-<?php echo esc_attr( $tag ); ?>" value="<?php echo esc_attr( $tag ); ?>" <?php echo ( in_array( $tag, $selected_tags ) ) ? 'checked="checked"' : ''; ?> /> <label for="post_tag-<?php echo esc_attr( $tag ); ?>"><?php echo esc_html( $label ); ?></label>
				<?php endforeach; ?>
			</fieldset>
		</div>
		<?php 
	}

	/**
	 * register the metabox with WordPress
	 *
	 */
	function pppf_register_custom_tag_metabox() {
		if ( current_user_can('edit_pages') ) return;
		add_meta_box( 'posttag-custom', _x( 'Tags', 'ppp-posttags-select-only' ), 'pppf_custom_tag_meta_box', null, 'side', '' );
	}

	add_action( 'add_meta_boxes', 'pppf_register_custom_tag_metabox' );
	
	function pppf_save_custom_tag( $post_id ) {
	
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return $post_id;
		if ( current_user_can( 'edit_pages', $post_id ) ) return $post_id;  
		if ( !current_user_can( 'edit_post', $post_id ) ) return $post_id;
		
		if ( !isset( $_POST['tax_input']['post_tag'] ) )
			wp_delete_object_term_relationships( $post_id, 'post_tag' );

	}
	
	add_action( 'save_post', 'pppf_save_custom_tag' );