Yourls2.php

<?php
/* ... */
	const RETURN_FORMAT_JSON = 'json';
	const RETURN_FORMAT_XML = 'xml';

	/**
	 * Generates new short URL
	 *
	 * @param string $url
	 * @param string $format
	 * 
	 * @return string|null
	 */
	public function createShortUrl( $url = '', $format = null )
	{
		$data = [
			'signature' => $this->token,
			'format'    => $format ?: self::RETURN_FORMAT_JSON,
			'action'    => 'shorturl',
			'url'       => $url
		];

		$result = $this->library->post( '/', $data );

		if ( isset($result) && isset($result->shorturl) )
		{
			return $result->shorturl;
		}
		else
		{
			return null;
		}
	}
/* ... */