The truth is when send_headerssend_headers
action is processing, the $wp_query$wp_query
isn't ready yestyet and you can't use is_single()is_single()
function. But you can use it a bit later. There is a parse_queryparse_query
action, that can help you. It starts when the $wp_query$wp_query
is ready and comes out right after the send_headerssend_headers
action.