Error when setting a constant

Question

I’m getting the following error in my code, and I’m not getting the reason why.

Got error 'PHP message: PHP Warning: Constants may only evaluate to scalar values in config.php on line 85

On the lines 81 to 85 I have the following:

define("DATA_CONFIG",[
   "master_id" => $dados_config->sistema_master_id,
   "sis_url" => $dados_config->sistema_url,
   "api_key" => base64_decode($dados_config->sistema_key)
]);

In the 85 I have the closing, but in the 84 I have the following line, there is some problem to use the base64_decode in a constant? Or making this conversion where I am doing may cause some problem?

"api_key" => base64_decode($dados_config->sistema_key)