thesaurus.php

<?php

//the Thesaurus PHP task from Test Dome

class Thesaurus
{
    private $thesaurus;

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

    public function getSynonyms($word)
    {
        $synonims = (!empty($this->thesaurus[$word]) ? $this->thesaurus[$word] : array());
        return json_encode(array('word'=>$word, 'synonyms'=>$synonims));
    }
}

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

echo $thesaurus->getSynonyms("big");
echo "\n";
echo $thesaurus->getSynonyms("agelast");

why don't you use the following shortcut, that is supported since php7:

{
   return json_encode(["word" => $word, "synonyms" => $this->thesaurus[$word] ?? []]);
}