WordPress function to

sjf_deh_get_image_src.php

<?php
/**
 * Get the current post featured image, with a call to wp_is_mobile to grab a smaller size for mobile users.
 *
 * This function uses wp_is_mobile(), which is not intended for public use on the front end -- buyer beware.
 * @return string|bool Returns the src for the featured image, or false on failure
 */
function sjf_deh_get_image_src(){

	// $post must work in order to grab the featured image
	$post = get_post();
	if( empty( $post ) ) { return false; }
	$post_id = absint( $post->ID );


	//we're trying to manipulate posts of the attachment type only
	if ( empty( $post->post_type ) || ( $post->post_type != 'attachment' ) ) { return false; } 

	// mobile users get a smaller version of the image
	// wp_is_mobile is not intended for use on the front end because it can go haywire if you use caching on your site.
	if( wp_is_mobile() ) {	
		$retina_img = wp_get_attachment_image_src( $post_id, 'sjf_deh_mobile_bg' );
	} else {
		$retina_img = wp_get_attachment_image_src( $post_id, 'sjf_deh_bg' );
	}
	
	// if there was no featured image, bail
	if( !$retina_img ) { return false; }

	// sanitize and return the image src
	$out = esc_url( $retina_img[0] );
	return $out;

}