serverOptimizedImageOnFly.php

<?php

class ImageResizer {

    private $cacheDir;
    private $cacheDirUri;

    /**
     * WPImageResizer constructor.
     *
     * @param string $cacheDir The cache directory where resized images will be stored
     * @param string $cacheDirUri The URL path to the cache directory
     */
    public function __construct($cacheDir, $cacheDirUri) {
        $this->cacheDir = $cacheDir;
        $this->cacheDirUri = $cacheDirUri;
    }

    /**
     * Resize an image and generate the <img> tag with srcset attribute for the resized images.
     *
     * @param string $imageUrl The URL of the original image
     * @param array $sizes An array of widths for the resized images
     * @return string The <img> tag with srcset attribute for the resized images
     * @throws \Exception If the image format is unsupported
     */
    public function renderOptimizedImage($imageUrl, $sizes) {
        $resizedImages = [];

        foreach ($sizes as $width) {
            $resizedImagePath = $this->cacheDir . '/' . $this->getCacheFilename($imageUrl, $width);

            if (!file_exists($resizedImagePath)) {
                $this->createResizedImage($imageUrl, $resizedImagePath, $width);
            }

            $resizedImages[] = $this->cacheDirUri . '/' . $this->getCacheFilename($imageUrl, $width);
        }

        return $this->generateImgTag($resizedImages, $sizes);
    }

    /**
     * Create a resized image based on the specified width.
     *
     * @param string $imageUrl The URL of the original image
     * @param string $resizedImagePath The path where the resized image will be saved
     * @param int $width The desired width of the resized image
     * @throws \Exception If the image format is unsupported
     */
    private function createResizedImage($imageUrl, $resizedImagePath, $width) {
        list($originalWidth, $originalHeight, $extension) = getimagesize($imageUrl);

        // Calculate the height based on the aspect ratio
        $height = (int)(($width / $originalWidth) * $originalHeight);

        // Load the image based on the file extension
        switch ($extension) {
            case IMAGETYPE_JPEG:
                $sourceImage = imagecreatefromjpeg($imageUrl);
                break;
            case IMAGETYPE_PNG:
                $sourceImage = imagecreatefrompng($imageUrl);
                break;
            case IMAGETYPE_GIF:
                $sourceImage = imagecreatefromgif($imageUrl);
                break;
            default:
                throw new \Exception('Unsupported image format');
        }

        // Create a new blank image with the desired dimensions
        $resizedImage = imagecreatetruecolor($width, $height);

        // Preserve transparency for PNG images
        if ($extension === IMAGETYPE_PNG) {
            imagealphablending($resizedImage, false);
            imagesavealpha($resizedImage, true);
            $transparent = imagecolorallocate($resizedImage, 0, 0, 0);
            imagecolortransparent($resizedImage, $transparent);
            imagefilledrectangle($resizedImage, 0, 0, $width, $height, $transparent);
        }

        // Set the image interpolation method to Bicubic for better quality
        imagesetinterpolation($resizedImage, IMG_BICUBIC);

        // Resize the image
        imagecopyresampled($resizedImage, $sourceImage, 0, 0, 0, 0, $width, $height, $originalWidth, $originalHeight);

        // Save the resized image
        switch ($extension) {
            case IMAGETYPE_JPEG:
                // Adjust the JPEG quality to your desired value (80 is just an example)
                imagejpeg($resizedImage, $resizedImagePath, 80);
                break;
            case IMAGETYPE_PNG:
                imagepng($resizedImage, $resizedImagePath);
                break;
            case IMAGETYPE_GIF:
                imagegif($resizedImage, $resizedImagePath);
                break;
        }

        imagedestroy($sourceImage);
        imagedestroy($resizedImage);
    }

    /**
     * Generate the <img> tag with srcset attribute for the resized images.
     *
     * @param array $resizedImages An array of paths to the resized images
     * @param array $sizes An array of widths for the resized images
     * @return string The <img> tag with srcset attribute
     */
    private function generateImgTag($resizedImages, $sizes) {
        $imgTag = '<img src="' . $resizedImages[0] . '" srcset="';

        foreach ($resizedImages as $index => $resizedImage) {
            $width = $sizes[$index];
            $imgTag .= $resizedImage . ' ' . $width . 'w, ';
        }

        $imgTag = rtrim($imgTag, ', ') . '">';

        return $imgTag;
    }

    /**
     * Get the cache filename for the resized image.
     *
     * @param string $imageUrl The URL of the original image
     * @param int $width The width of the resized image
     * @return string The cache filename
     */
    private function getCacheFilename($imageUrl, $width) {
        $pathInfo = pathinfo($imageUrl);
        $extension = strtolower($pathInfo['extension']);
        $filename = $pathInfo['filename'];

        return $filename . '-' . $width . 'x.' . $extension;
    }
}