A basic responsive image plugin for Kirby

responsiveImage.php

<?php

function responsiveImage($image, $classNames = [], $sizes = '100vw', $upscale = false) {
	$widths = [2800, 1400, 1000, 600, 400, 100];
	$imageWidth = $image->width();
	$defaultClass = 'srcset-image';
	if (gettype($classNames) === 'string') $classNames = explode(' ', $classNames);
	array_push($classNames, $defaultClass);
	$classString = join($classNames, ' ');

	$src = ($upscale) ? thumb($image, array('upscale' => true, 'width' => $widths[0]), false) : $image->url();
	$srcset = [];
	for ($i = 0; $i < count($widths); $i++) {
		$newSrc = false;
		$width = $widths[$i];
		$nextWidth = ($i < count($widths) - 1) ? $widths[$i + 1] : false;
		if ($upscale == true) {
			$newSrc = thumb($image, array('upscale' => $upscale, 'width' => $width), false) . " " . $width . "w";
		} else if ($imageWidth > $width) {
			$newSrc = thumb($image, array('upscale' => $upscale, 'width' => $width), false) . " " . $width . "w";
		} else if ($imageWidth > $nextWidth) {
			$newSrc = thumb($image, array('upscale' => $upscale, 'width' => $imageWidth), false) . " " . $imageWidth . "w";
		}
		if ($newSrc) array_push($srcset, $newSrc);
	}
	$srcset = join($srcset, ', ');
	return ('
	<img class="' . $classString . '"
			 src="' . $src . '"
			 srcset="' . $srcset . '"
			 sizes="' . $sizes . '"
			 alt="' . $image->caption() . '" />
	');
}

?>