I was wondering if I should use "meta tags" in my php script to "activate" keywords when I list my site at google, altavista... Or should I use something else, in order to have some "usefull" keywords.?
I use meta description tags to get a sensible keyword-rich description in the search results, but I don't usually use keyword tags. A good (non-spammy) meta description can increase the number of people who click your listing dramatically.
If your site has been indexed by googlebot don't add meta keywords sitewide all at once, it might trip a spam filter.
You should instead focus on getting your keywords into your page text and headers in a non-spammy way.