If you're working with functions that are not part of the core of PHP, that is, functions that are from an extension that needs to be enabled by users, it's a smart move to use the function_exists() function. This takes a function name as its only parameter, and returns true if that function (either built-in, or one you've defined yourself) is available for use. Note that it only checks whether the function is available, not whether it will work - your system may not be configured properly for some functions.
Author's Note: If you ever want to know whether you have a function available to you, use the function_exists() function. This takes one string parameter that is the name of a function, and returns true if the function exists or false if it does not. Many people use function_exists() to find out whether they have an extension available, by calling function_exists() on a function of that extension. However, this is accomplished much more easily with the function extension_loaded() function covered later.