Parse error when using function in array

I'm trying to create an array of functions. Although, I'm getting parse error and nothing more when executing this code

protected $filter_functions = [
    "price" => function(&$query, $lower, $higher) {
        $query->where("price", ">=", $lower)->where("price", "<=", $higher);
    }
];

I can't see any syntax error but maybe there is. The PHP version is currently 5.6.28 and should be compatible with anonymous functions.

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