src/Certificate/FileCertificateProvider.php
protected function retrieveCertificateFromStore($certificateChainUri)
    {
        try {
            $certificateChain = $this->filesystem->get($this->calculateFilePath($certificateChainUri));
        } catch (FileNotFoundException $e) {
            $certificateChain = null;
        }

        return $certificateChain;
    }
protected function retrieveCertificateFromStore($certificateChainUri)
    {
        try {
            $certificateChain = $this->filesystem->get($this->calculateFilePath($certificateChainUri));
        } catch (FileNotFoundException $e) {
            $certificateChain = null;
        }

        return $certificateChain;
    }
Retrieve the certificate give the certificate chain's uri from the datastore

@param string $certificateChainUri

@return string|null