PHP的json_decode使用elasticsearch查询结果返回NULL

Question

I've seen similar question here, but no one with an aggregation which might have something to do with the problem at hand.

This is the code I'm trying to run:

require '../vendor/autoload.php';

$client = Elasticsearch\ClientBuilder::create()->build();

$params = [
    'index' => 'search_log',
    'type' => 'search_log',
    'body' => [
        'size' => 0,
        'aggs' => [
            'popular_terms' => [
                'terms' => [
                    'field' => 'filteredSearch'
                ]
            ]
        ]
    ]
];

// Execute query
$response = $client->search($params);

// Decode response
$myData = json_decode($response);
var_dump($myData);

The output I get:

NULL

I have the problem narrowed down to the decoding part. If I execute a print_r of the $response, this is what I get (actually all in one line, I've pretty-printed it manually so it's easier to read):

Array (
    [took] => 245
    [timed_out] =>
    [_shards] => Array (
        [total] => 5
        [successful] => 5
        [failed] => 0
    )
    [hits] => Array (
        [total] => 5124004
        [max_score] => 0
        [hits] => Array ( )
    )
    [aggregations] => Array (
        [popular_terms] => Array (
            [doc_count_error_upper_bound] => 37750
            [sum_other_doc_count] => 11388032
            [buckets] => Array (
                [0] => Array (
                    [key] => term1
                    [doc_count] => 385107
                )
                [1] => Array (
                    [key] => term2
                    [doc_count] => 169381
                )
                [2] => Array (
                    [key] => term3
                    [doc_count] => 155258
                )
                [3] => Array (
                    [key] => term4
                    [doc_count] => 150382
                )
                [4] => Array (
                    [key] => term5
                    [doc_count] => 124759
                )
                [5] => Array (
                    [key] => term6
                    [doc_count] => 102589
                )
                [6] => Array (
                    [key] => term7
                    [doc_count] => 98791
                )
                [7] => Array (
                    [key] => term8
                    [doc_count] => 98772
                )
                [8] => Array (
                    [key] => term9
                    [doc_count] => 98091
                )
                [9] => Array (
                    [key] => term10
                    [doc_count] => 94559
                )
            )
        )
    )
)

My desired result would be to print just term1, term2, etc, without the rest of stuff that comes with Elasticsearch responses.