I would simply do a string_replace if it doesn't require any complicated logic.
str_replace('<embed', '<embed type="application/x-shockwave-flash" and_anything_else_you_want_added ', $string)
Make sure there is a space after what's added, like I did.
Also although this might be off-topic, you really shouldn't store HTML like that, because you run into maintenance issues like this. HTML should be generated as part of the display. The only real exception is if it takes heavy processing power and you need to cache the HTML, in which case you store the raw data (so it's easy to regenerate the HTML) with the cached (so it's quicker to output).