extension_loaded

Finds out whether the extension is loaded.

The extension name. This parameter is case-insensitive.

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]

Returns TRUE if the extension identified by name is loaded, FALSE otherwise.

<?php
if (!extension_loaded('gd')) {
    if (!dl('gd.so')) {
        exit;
    }
}
?>

get_loaded_extensions() -

get_extension_funcs() -

phpinfo() -

dl() -

function_exists() -