A special note needs to be made about the function_score query. Due to the way PHP handles JSON encoding, everything is converted to an array of one for or another. This is usually not a problem, since most places in the Elasticsearch API accept arrays or empty objects interchangeably.

However, the function_score is a little different and needs to differentiate between empty arrays and empty objects. For example, consider this query:

{
   "query":{
      "function_score":{
         "functions":[
            {
               "random_score":{}
            }
         ],
         "boost_mode":"replace",
         "query":{
            "match_all":{}
         }
      }
   }
}

The function_score defines an array of objects, and the random_score key has an empty object as it’s value. PHP’s json_encode will convert that query to this:

{
   "query":{
      "function_score":{
         "functions":[
            {
               "random_score":[]
            }
         ],
         "boost_mode":"replace",
         "query":{
            "match_all":[]
         }
      }
   }
}

Which will result in a parse exception from Elasticsearch. What we need to do is tell PHP that random_score contains an empty object, not an array. To do so, we need to specify an explicitly empty object:

$params['body'] = array(
    'query' => array(
        'function_score' => array(
            'functions' => array(
                array("random_score" => (object) array())
            ),
            'query' => array('match_all' => array())
        )
    )
);
$results = $client->search($params);

Now, the JSON will be encoded properly and your query will no longer generate a parser exception.