//php
function change_subject(){
	$return = array(
		'message'	=> 'Saved',
		'ID'		=> 1
	);

	wp_send_json_success($return);
}
add_action( 'wp_ajax_change_subject', 'change_subject' );
add_action( 'wp_ajax_nopriv_change_subject', 'change_subject' );

//js
$.post( ajax_search_filters_data.ajaxurl, {
	    	'action' : 'change_subject_callback'
	    },
	    function( response ){
	    	console.log(response); //outputs 0
	    },
	    'json');