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<?php
/* 2020-02-19 */
add_action( 'add_meta_boxes',  function() { 
    add_meta_box('html_myid_61_section', 'Напрямки роботи лікаря', 'my_output_function');
 });

function my_output_function( $post ) {
    wp_editor( htmlspecialchars_decode( get_post_meta($post, 'SMTH_METANAME' , true ) ), 'sdstudio_napryamku_robotu_doctora', $settings = array('textarea_name'=>'MyInputNAME') );
}

add_action( 'save_post', function($post_id) {
    if (!empty($_POST['MyInputNAME'])) {
        $datta=htmlspecialchars($_POST['MyInputNAME']);
        update_post_meta($post_id, 'SMTH_METANAME', $datta );
    }
});