Teste

<?php

class Thesaurus
{
    private $thesaurus;

    function Thesaurus($thesaurus)
    {
        $this->thesaurus = $thesaurus;
    }

    public function getSynonyms(string $word)
    {
        if($this->thesaurus[$word]) {
            $data = [
                'word' => $word,
                'synonyms' => $this->thesaurus[$word]
            ];

        } else {
            $data = [
                'word' => $word,
                'synonyms' => []
            ];
        }

        return json_encode($data);
    }
}

$thesaurus = new Thesaurus(
    array
        (
            "buy" => array("purchase"),
            "big" => array("great", "large")
        ));

echo $thesaurus->getSynonyms("big");