srcset image tag functions for WordPress

srcset.php

// Image Sizes
add_image_size( 'padmini', 1024 );
add_image_size( 'phone', 1136 );
add_image_size( 'phoneplus', 1334 );
add_image_size( 'padretina', 2048 );

function spellerberg_get_image($imageid,$fallbacksize = 'full') {

	$sizes[] = wp_get_attachment_image_src( $imageid, 'padmini');
	$sizes[] = wp_get_attachment_image_src( $imageid, 'phone');
	$sizes[] = wp_get_attachment_image_src( $imageid, 'phoneplus');
	$sizes[] = wp_get_attachment_image_src( $imageid, 'padretina');
	$sizes[] = wp_get_attachment_image_src( $imageid, 'full');
	
	$srcset = '';

	foreach ( $sizes as $size ) :
		$srcset .= $size[0] . ' ' . $size[1] . 'w, ';
	endforeach;
	
	$fallback = wp_get_attachment_image_src( $imageid, $fallbacksize);
	
	$alt_text = get_post_meta($imageid, '_wp_attachment_image_alt', true);
	
	if ( !$alt_text || $alt_text == "" ) :
		$attachment = get_post($imageid);
		$alt_text = $attachment->post_title;
	endif;
	
	$output = '<img ';
	
	if ( $srcset != '' ) :
		$output .= 'srcset="' . $srcset . '" ';
	endif;
	
	$output .= 'src="' . $fallback[0] . '" ';
	$output .= 'alt="' . $alt_text . '">';
	
	return $output;

}


function spellerberg_the_image($imageid,$fallbacksize = 'full') {
	echo spellerberg_get_image($imageid,$fallbacksize);
}

function spellerberg_get_thumbnail($post_id, $fallbacksize = 'full') {
	$imageid = get_post_thumbnail_id( $post_id );
	if ( $imageid ) :
		return spellerberg_get_image($imageid,$fallbacksize);
	endif;
}

function spellerberg_the_thumbnail($post_id, $fallbacksize = 'full') {
	echo spellerberg_get_thumbnail($post_id, $fallbacksize);
}