2 php questions

I have 2 questions about php.

1st question

can i use php in meta keyword and description tags. example i have a database full of keywords can i use php to print out my keywords.) I have done this with the title but im not sure if it works with the meta tags.

2nd question

is there a limit to how many times i can run php on my page before it blows up. I have a few includes but i would rather use ssi’s but not sure if my server supports it.