You can create your own libraries with functions defined in PHP by implementing the MadLisp\Lib\ILib interface. The register method takes an environment as argument where functions can be set. The interface is defined like this:

namespace MadLisp\Lib;

use MadLisp\Env;

interface ILib
    public function register(Env $env): void;

You can then register your library with something like this:

$factory = new MadLisp\LispFactory();
$lisp = $factory->make();

$myLib = new My\Library();

Take a look at the built-in libraries for examples.