Get the featured image url. For situations where you just need the img src of the featured image. Use the function provided for easy usage.

get_featured_image_url.php

$fi_src = wp_get_attachment_image_src( get_post_thumbnail_id($post_id), 'thumbnail_size' );
$fi_url = $fi_src[0];

// Returns featured image url, optional size argument.
// Use like this: get_featured_image_url('thumbnail');
function get_featured_image_url( $size = "full" ) {
	global $post;
	
	$fi_src = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), $size );
	$fi_url = $fi_src[0];

	return $fi_url;
}