Generate a img tag for responsive images in WordPress

get-responsive-image.php

<?php
/**
 * Get the responsive image.
 *
 * @param string $image_id 
 * @param string $image_size optional
 *
 * @return string $output
 */
function get_responsive_image( $image_id, $size = 'medium' ) {
	
	// Check the image id
	if( ! absint( $image_id ) ) {
		return false;
	}

	// Check the attachment exists
	$img_src = wp_get_attachment_image_url( $image_id, $size );	
	if ( ! $img_src ) {
		return false;
	}
	
	// Gather the required elements
	$img_srcset = wp_get_attachment_image_srcset( $image_id, $size );
	$img_sizes  = wp_get_attachment_image_sizes( $image_id, $size );
	$image_alt  = get_post_meta( $image_id, '_wp_attachment_image_alt', true );
	$image_alt  = empty( $image_alt ) ? '' : ' alt="' . esc_attr( $image_alt ) .'"';
	
	// Build the output.
	$output = '<img src="' . esc_url( $img_src ) . '"
		 srcset="' . esc_attr( $img_srcset ) . '"
		 sizes="' . esc_attr( $img_sizes ) . '"' . $image_alt . '>';
	
	/**
	 * Filter the responsive image output.
	 *
	 * @param string $output   the responsive image html.
	 * @param int    $image_id the assoicated image id.
	 * @param mixed  $size     the image size, string or array.
	 * @return string $output
	 */
	return apply_filters( 'get_responsive_image', $output, $image_id, $size );
}

Thanks for the feedback! I've updated the gist accordingly, also added a apply_filters to the output.