Add item to collection/PHP array with condition

Hello,

I'm using laravel, and I have array to create, depends on some condition

Ex :

$query = [
    'query' => [
        'query_string' => [
            "query" => "ibis"
        ]
    ]
];

and I have :

if (request()->has('category')) {
    // I want to add for ex ['category' => request('category')]
}

So the final result should be :

$query = [
    'query' => [
        'query_string' => [
            "query" => "ibis",
            "category" => "some value"
        ]
    ]
];

I can use Collection or Array, but how ?

Thanks