Highlight, number_of_fragments seem does not work

Hello,
I'm using this elasticsearch client
: https://github.com/nervetattoo/elasticsearch
And I have no "highlight" result when I search :

$params = array(
"index" =>"my_index",
"type" => "my_type",
"body" => array(
"size"=>30,
'query' => array(
'multi_match' => array(
'fields' => array('title','description')
'query' => keywords,
),
),
)
);

   * 

$params['body']['highlight']['fields'][$value]['number_of_fragments']=20; *// not
work, I don't know why ?

$results = $this->elasticsearchClient->search($params);

Can you help me ?
Thank you in advance.

Tanguy

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