(PHP 4, PHP 5, PHP 7, PHP 8)
extension_loaded — Find out whether an extension is loaded
$extension): boolFinds out whether the extension is loaded.
extensionThe 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 extension
   is loaded, false otherwise.
  
Example #1 extension_loaded() example
<?php
if (!extension_loaded('gd')) {
    if (!dl('gd.so')) {
        exit;
    }
}
?>