To disable email notification after password change, Add the following code in your theme file or plugin files.

/* To disable email notification after password change */
add_filter( 'send_password_change_email', '__return_false' );

Please feel free to contact me if you need any further information.

By default WordPress having an function called get_cat_ID() to retrieve get category ID from category name. Please check the below sample code for how to get category ID from category name in WordPress. Replace ‘Category Name’ with your category name.

$categoryID = get_cat_ID('Category Name');

If you have any doubt, Please comment here.

We can retrieve list of all sites in WordPress Multisite Network by using get_sites() function.
This function will return the data in array format. Please check the sample code below.

$sites = get_sites();
var_dump($sites);

If you have any doubt, Please comment here.

We can retrieve list of themes with theme data by using wp_get_themes() function.
This function will return the data in array. Please check the sample code below.

$themes = wp_get_themes();
var_dump($themes);

If you have any doubt, Please comment here.