The extension name.
You can see the names of various extensions by using
phpinfo() or if you're using the
CGI or CLI version of
PHP you can use the
-m switch to
list all available extensions:
$ php -m [PHP Modules] xml tokenizer standard sockets session posix pcre overload mysql mbstring ctype [Zend Modules]
Note: extension_loaded() uses the internal extension name to test whether a certain extension is available or not. Most internal extension names are written in lower case but there may be extension available which also use uppercase letters. Be warned that this function compares case sensitive !