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<?php
/**
 * Create an image with the desired size on-the-fly.
 * 
 * @param $image_id
 * @param $width
 * @param $height
 * @param $crop
 *
 * @return array
 */
function lazy_image_size($image_id, $width, $height, $crop) {
    // Temporarily create an image size
    $size_id = 'lazy_' . $width . 'x' .$height . '_' . ((string) $crop);
    add_image_size($size_id, $width, $height, $crop);

    // Get the attachment data
    $meta = wp_get_attachment_metadata($image_id);

    // If the size does not exist
    if(!isset($meta['sizes'][$size_id])) {
        require_once(ABSPATH . 'wp-admin/includes/image.php');

        $file = get_attached_file($image_id);
        $new_meta = wp_generate_attachment_metadata($image_id, $file);

        // Merge the sizes so we don't lose already generated sizes
        $new_meta['sizes'] = array_merge($meta['sizes'], $new_meta['sizes']);

        // Update the meta data
        wp_update_attachment_metadata($image_id, $new_meta);
    }

    // Fetch the sized image
    $sized = wp_get_attachment_image_src($image_id, $size_id);

    // Remove the image size so new images won't be created in this size automatically
    remove_image_size($size_id);
    return $sized;
}