Get array of post IDs

gistfile1.php

<?php

$featured_posts = get_posts(
	array( 
		'posts_per_page' => 5, 
		'tax_query'     => array(
		    array(
			'taxonomy'  => 'post_tag',
			'field'     => 'slug',
			'terms'     => 'featured',
		    ),
		),
		'fields' => 'ids'
	)
);