My robots.txt contains the line
Shall I nevertheless include
http://example.com/search.php into my sitemap?
Webmasters Stack Exchange is a question and answer site for pro webmasters. It only takes a minute to sign up.Sign up to join this community
You should not add URLs to your sitemap unless you want them crawled and indexed. If a URL is disallowed, non-canonical, or redirecting, it doesn't belong in your XML sitemap.
Google uses your XML sitemap as a list of pages that it should crawl. When you include non-crawlable pages in your XML sitemap Google will complain about them in Google Search Console. You would get the warning "Sitemap Contains Blocked Resources."
For further reading see my answer to How should I choose which URLs I want indexed to include in my sitemap?
You are correct to disallow search because Google doesn't want to index other search results. It is bad user experience for users to click from Google search result to your site search results. Google may penalize sites that allow their site search results to be crawled and indexed. See Matt Cutts: Search results in search results.