Certificate not accepted. Unable to set private key file

I try to make a connection true SoapClient. I need a certificate for this. I received a .pfx certificate. I used the following command to create a .pem file.

openssl pkcs12 -in cert.pfx -out cert.pem -nodes

There is a password in the certificate so i need to enter it before i get the cert.pem file. So far so good, i think.

Now i try to connect to the wdsl service.

$url = "https://test.website.com/webservices/transfer.asmx?WSDL";
$cert = '/path/to/cert.pem';
$passphrase = "12345678";                                               

$soapClient = new SoapClient($url, array('local_cert'=>  $cert,'passphrase'=>$passphrase));

I get the following error:

(Warning) SoapClient::SoapClient(): Unable to set private key file `/var/www/vhosts/............./cert.pem'

I think te problem is the certificate. Is the way that i convert the .pfx to a .pem the correct way?