Are there Libraries which are in every present PHP installation?
An Example:
I'm using cURL in a PHP Script, does it run on every PHP installation?
Libraries like cURL aren't in a default PHP installation. Though most hosts do carry a default set of extensions which have been widely used, which usually include cURL.
PHP has a page on which it's built-in functions which will most likely explain this further: http://nl3.php.net/manual/en/functions.internal.php
There's also a page with function references to find the most used libraries that can be installed using PEAR: http://nl3.php.net/manual/en/funcref.php