Dynamic PHP pages and SEO

Guys,
Been racking my brain on this and can't figure it out: I have a website which uses a MySQL DB to create a dynamic page. The page URL is www.example.com/contentdetail.php?id=456. So, based on the id, you could see different content on the page.
Q: How do I make Google crawl the website and think that each id instance is a new page? (This is off the assumption that more content = good). I am really lost here since none of these pages are being indexed right now. Do I have to link them to the main page for the little spider to look? Any help is appreciated!

They have to be found somehow.. usually from another page

Create a sitemap and submit it to Google using WMT

If you want to get better SEO benefits you should make the URLs readable and include keywords using .htaccess redirects

Also use a mod_rewrite via htaccess to create dynamic looking pages such as example.com/content/124.htm will rewrite to examples.com/contentdetail.php?id=124 without the end user realizing.

Mod rewrite won't tell Google where the pages are. You'll need a sitemap or at least link to each of your pages from different parts of your site.