PHP SoapClient SSL: Failed to load external entity (certificate not loaded)

I am connecting to a WSDL which needs a SSL file. I created this php script to connect to the SOAP API:

$url = "https://www.testsoap.nl/api/soap?wsdl";

$soapArr = array(
    "soap_version"=>SOAP_1_1,
    "trace"=>true,
    "exceptions"=>true,
    "local_cert"=>"certfile.p12",
    "passphrase"=>"passwordhere"
);

$client = new SoapClient($url, $soapArr);

Now when I try to connect to I will see this error in my error log:
[Fri Jan 10 11:08:21 2014] [error] [client 172.21.1.131] PHP Fatal error: Uncaught SoapFault exception: [WSDL] SOAP-ERROR: Parsing WSDL: Couldn't load from 'https://www.testsoap.nl/api/soap?wsdl' : failed to load external entity "https://www.testsoap.nl/api/soap?wsdl"\n in /var/www/index.php:39\nStack trace:\n#0 /var/www/index.php(39): SoapClient->SoapClient('https://www.tes...', Array)\n#1 {main}\n thrown in /var/www/index.php on line 39

Line 39 = $client = new SoapClient($url, $soapArr);

I also tried these array options, they did not change anything however:

$soapArr = array(
    "style"=>SOAP_DOCUMENT,
    "use"=>SOAP_LITERAL,
    "soap_version"=>SOAP_1_1,
    "trace"=>true,
    "authentication"=>SOAP_AUTHENTICATION_DIGEST,
    "exceptions"=>true,
    "local_cert"=>"certfile.p12",
    "passphrase"=>"passwordhere",
    "ssl_method"=>SOAP_SSL_METHOD_TLS
);

Some things to consider:

• I can install this SSL cert file in my browser and connect to the API without any problem.

• When using soapUI (with this the password and the certificate) I can also connect to the API

• openssl is installed on my debian server

• the debianserver can connect to the internet

• php-soap is installed on the debian server

I did some more testing and found out that I get the same errors when I do not use any certificate like this:

$client = new SoapClient($url);

So I must assume the certificate is not loaded like I would expect.

I found this bug: https://bugs.php.net/bug.php?id=27777
And here: http://www.php.net/manual/en/soapclient.soapclient.php#83474 Somebody says this also counts for the SSL certificates.

However, reading this information and the possible solution, I am still not any further…

Any help would be much appreciated.
Thanks in advance!