get-post-idby-term.php

<?php
/*
 * Returns post IDs in a term with a given ID
 *
 * @param int $term_id(Category ID, Tag ID)
 *
 * @return array|int Post ID(s)
 */
function get_post_id_by_term_id( $term_id ){
	global $wpdb;
 
	$ids = $wpdb->get_col( $wpdb->prepare( "SELECT object_id FROM $wpdb->term_relationships WHERE term_taxonomy_id = %d", $term_id ) );
 
	if( count( $ids ) > 1 ) 
		return $ids; // return array
	else
		return $ids[0]; // return int
}
?>