How to check if mcrypt extension exists in php

You can also use extension_loaded():

if (extension_loaded('mcrypt')) {
    echo "mcrypt is loaded!";
} else {
    echo "mcrypt isn't loaded!";
}

You can use function_exists to check if one of the mcrypt functions exists.

if(function_exists('mcrypt_encrypt')) {
    echo "mcrypt is loaded!";
} else {
    echo "mcrypt isn't loaded!";
}

Edit 30.07.2016:
Since my answer still gets a few upvotes from time to time, I benchmarked the performance of mine and Cristi Draghici's answers. The conclusion is, that function_exists is a bit faster than extension_loaded. https://3v4l.org/So4Ep

If you are using a development environment like XXAMP, or WAMP, there should be a default "phpinfo" page. For example, in XXAMP it would be:

http://localhost/dashboard/phpinfo.php

You can also achieve this same screen by viewing a php file that has: phpinfo(); somewhere in the code.

In this screen, simply search for the string "mcrypt support". If installed, you will see a box that says "enabled".