add-custom-field-to-product.php

<?php

add_action( 'woocommerce_product_options_general_product_data', 'toret_custom_product_field' );
function toret_custom_product_field(){

  woocommerce_wp_text_input( 
	   array( 
		  'id'                => 'custom_product_text', 
		  'label'             => __( 'Produkt text', 'toret-skoleni' ), 
		  'placeholder'       => 'Produkt text', 
      'desc_tip'          => 'true',
		  'description'       => __( 'Produkt text', 'toret-skoleni' ) 
	    )
   );

}

add_action('save_post', 'woo_add_custom_general_fields_save' );
function woo_add_custom_general_fields_save( $post_id ){
	
    //Check autosave
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return $post_id;
    }
    
    //Don't update on Quick Edit
    if ( defined( 'DOING_AJAX') ) {
        return $post_id;
    }
    
    //Don't update on Bulk Edit
    if ( !empty( $_REQUEST['bulk_edit'] ) ) {
        return $post_id;
    }

    if( get_post_type( $post_id ) != 'product' ){
        return $post_id;
    }
  
  if( !empty( $_POST['custom_product_text'] ) ){
    update_post_meta( $post_id, 'custom_product_text', $_POST['custom_product_text'] );
  }

}

add_action( 'woocommerce_single_product_summary', 'toret_display_meta_data' , 12 );
function toret_display_meta_data(){
  global $post;
  $text = get_post_meta( $post->ID, 'custom_product_text', true );
  echo '<p>'.$text.'</p>';
}