add-cta.php

<?php
class Add_CTA_To_Content {

	/**
	 * Call To Action to add to post
	 *
	 * @var string
	 */
	protected $call_to_action;

	/**
	 * Add_CTA_To_Content constructor.
	 *
	 * @param  string $call_to_action Call To Action to add to post
	 */
	public function __construct( $call_to_action ){
		$this->call_to_action = $call_to_action;
	}

	/**
	 * Add the filter
	 */
	public function add_hook(){
		add_filter( 'the_content', [ $this, 'filter_content' ] );
	}

	/**
	 * Remove the filter
	 */
	public function remove_hook(){
		remove_filter( 'the_content', [ $this, 'filter_content' ] );

	}

	/**
	 * Append CTA to post content
	 *
	 * @param string $content
	 *
	 * @return string
	 */
	public function filter_content( $content ){
		return $content . $this->call_to_action;
	}

}

save-cta.php

<?php
add_action( 'save_post', 'save_cta', 10, 2 );
function save_cta( $id, $post ){
	remove_action( 'save_post', 'save_cta' );
	$appender = new Add_CTA_To_Content( get_option( 'saved_cta' ) );
	$post->post_content = $appender->filter_content( $post->post_content );
	wp_update_post( $post );

}

static-1.php

<?php
add_action( 'save_post', [ 'Term_Control_Hooks', 'save_post' ], 10, 2 );
class Term_Control_Hooks {

	public static function save_post( $id, $post  ){
		if( ! empty( $_POST[ 'post_category' ] ) ){
			$categories = $_POST[ 'post_category' ];
			$control = new Term_Control_2( $post, $categories );
			$control->update_categories();
		}
	}
}

static-2.php

<?php
add_action( 'admin_init', [ 'Term_Control_Hooks', 'admin_hooks' ] );


class Term_Control_Hooks {

	public static function admin_hooks(){
		add_action( 'save_post', [ __CLASS__, 'save_post' ], 10, 2 );

	}

	public static function save_post( $id, $post  ){
		if( ! empty( $_POST[ 'post_category' ] ) ){
			$categories = $_POST[ 'post_category' ];
			$control = new Term_Control_2( $post, $categories );
			$control->update_categories();
		}
	}
	
}

static-3.php

<?php
add_action( 'admin_init', [ 'Term_Control_Hooks', 'admin_hooks' ] );


class Term_Control_Hooks {

	public static function admin_hooks(){
		add_action( 'save_post', [ __CLASS__, 'save_post' ], 10, 2 );
		add_action( 'admin_enqueue_scripts', [ __CLASS__ , 'load_scripts' ] );
	}

	public static function save_post( $id, $post  ){
		if( ! empty( $_POST[ 'post_category' ] ) ){
			$categories = $_POST[ 'post_category' ];
			$control = new Term_Control_2( $post, $categories );
			$control->update_categories();
		}
	}

	public static function load_scripts(){

	}

}

term-control-1.php

<?php
class Term_Control_1 {

	/**
	 * Create object
	 */
	public function __construct(){
		add_action( 'save_post', [ $this, 'save_post' ], 10 );
	}

	/**
	 * When post is updated, check if has category 7 or not and add or remove custom field
	 *
	 * @param $id
	 */
	public function save_post( $id ){
		remove_action( 'save_post', [ $this, 'save_post' ] );
		if( ! empty( $_POST[ 'post_category' ] ) && in_array( 7, $_POST[ 'post_category' ] ) ){
			update_post_meta( $id, '_fancy_cats', 1 );
		}else{
			delete_post_meta( $id, '_fancy_cats', 1 );

		}
		
	}

}

new Term_Control_1();

term-control-2-hook.php

<?php
add_action( 'save_post', 'my_update_post', 10, 2 );
function my_update_post( $id, $post ){
	if( ! empty( $_POST[ 'post_category' ] ) ){
		$categories = $_POST[ 'post_category' ];
		$control = new Term_Control_2( $post, $categories );
		$control->update_categories();
	}
	
}

term-control-2.php

<?php
class Term_Control_2 {

	/**
	 * @var array
	 */
	protected $categories;

	/**
	 * @var WP_Post
	 */
	protected $post;

	/**
	 * Category to check for
	 *
	 * @var int
	 */
	protected $category = 7;

	/**
	 * Custom field to write
	 *
	 * @var string
	 */
	protected $field = '_fancy_cats';

	/**
	 * Term_Control_2 constructor.
	 *
	 * @param WP_Post $post
	 * @param array $categories
	 */
	public function __construct( WP_Post $post, array $categories ){
		$this->post = $post;
		$this->categories = $categories;
	}

	/**
	 * Update categories
	 */
	public function update_categories(){
		if( $this->has_category() ){
			$this->add_field();
		}else{
			$this->remove_field();
		}
	}

	/**
	 * Add the custom field
	 */
	public function add_field(){
		update_post_meta( $this->post->ID, $this->field, 1 );
	}

	/**
	 * Remove the custom field
	 */
	public function remove_field(){
		delete_post_meta( $this->post->ID, $this->field, 1 );

	}

	/**
	 * Check if post has categories
	 *
	 * @return bool
	 */
	protected function has_category(){
		return in_array( $this->category, $this->categories );
	}


}