PHP and Multiple databases

I am fairly new to PHP and have used it on a few sites but I have not been dealing with databases much (in PHP). I am wanting to know if a single instance of PHP can be configured to connect to multiple databases or does is a new instance required for each DB. The scenario is that I may be pulling content from multiple databases (Oracle, MySQL, SQL Server...) for a single web site. I am not looking for any involved explanations just a simple yes or no and any reference points you may have from experience on how (at a high level) and if there are any "tricky parts" that might make the process painful. Thanks, Scott