How to access files from .env

MQTT_CA_CERT_PATH_PROD=/../../Cert/prod/ca-chain.crt

MQTT_CA_CERT_PATH_PROD=/../../Cert/prod/ca-chain.crt

i have given a path to a cert directory and accessing it in my config file

'MQTT_CA_CERT_PATH' => env('MQTT_CA_CERT_PATH_PROD'),

'MQTT_CA_CERT_PATH' => env('MQTT_CA_CERT_PATH_PROD'),

but gives an error .....
how to access that