Elgg\EntityIconService::getFallbackIconUrl PHP Method

    public function getFallbackIconUrl(ElggEntity $entity, array $params = [])
    {
        $type = elgg_extract('type', $params) ?: 'icon';
        $size = elgg_extract('size', $params) ?: 'medium';
        $entity_type = $entity->getType();
        $entity_subtype = $entity->getSubtype() ?: 'default';
        $exts = ['svg', 'gif', 'png', 'jpg'];
        foreach ($exts as $ext) {
            if ($ext == 'svg' && elgg_view_exists("{$type}/{$entity_type}/{$entity_subtype}.svg")) {
                return elgg_get_simplecache_url("{$type}/{$entity_type}/{$entity_subtype}.svg");
            }
            if (elgg_view_exists("{$type}/{$entity_type}/{$entity_subtype}/{$size}.{$ext}")) {
                return elgg_get_simplecache_url("{$type}/{$entity_type}/{$entity_subtype}/{$size}.{$ext}");
            }
        }
        if (elgg_view_exists("{$type}/default/{$size}.png")) {
            return elgg_get_simplecache_url("{$type}/default/{$size}.png");
        }
    }