By far the best resource for finding information about PHP and all of the functions available is from the PHP website. Not only can you view the PHP documentation, but you can also download PHP and many of the extensions like the Smarty template system.
Each PHP function and section has its own page with lots of detailed information about usage and instillation, which can be found quite easily on the site by entering the domain name followed by the function name you want to look up. If the function isn't found that the site points you towards a search results page.
One of the better aspects of the site is the user submitted code snippets that can be found on many of the pages. Take the substr() function as an example. There are many pages of user submitted functions, hints, tips, error trapping and other code snippets which can usually solve most of the problems that you are stuck on.
Although the instillation section is completely brilliant, the site probably isn't the best site in the world if you are starting out to learn PHP, there is a getting started section, but this wont get you past a certain level.