Handling filter calls from WordPress and plugins

add_filter( 'wp_body_open', 'my_wp_body_open', 10, 1 );    //FilterName, CallbackFunction, Priority (default10), Number of arguments expected (default/min 1)
//add_filter( 'wp_body_open', '\MyNamespaceName\my_wp_body_open', 10, 1 );   //<<<If you are using a namespace
function my_wp_body_open($ValueToFilter)
{

}