PHP SoapClient Warning: SoapClient::SoapClient(): Unable to set private key file

[FL]

I'm trying to use soap to call a webservice but I keep getting the following error "Warning: SoapClient::SoapClient(): Unable to set private key file".I'm assuming that the error comes due to the fact the the .cer file I am using only includes public key and no private key. But i'm not sure of another way to use the .cer file. If i don't use the .cer file i can connect just fine and I am able to call and receive results when i use the \[code\]__getFunctions()\[/code\] method. However, when i try to use other methods i need to be authorized and that leads to the problem. Below is the simple code i am trying to use. Please let me know if more information is required.\[code\]ini_set('display_errors',1);error_reporting(E_ALL);ini_set('soap.wsdl_cache_enabled', 0);$username = 'user';$password = 'pass';$ns = 'GatewayEDI.WebServices';$auth = array();$auth['User'] = new SOAPVar($username, XSD_STRING, null, null, null, $ns);$auth['Password'] = new SOAPVar($password, XSD_STRING, null, null, null, $ns);$headerBody = new SOAPVar($auth, SOAP_ENC_OBJECT);$header = new SOAPHeader($ns, 'AuthSOAPHeader', $headerBody);$client=new SoapClient('https://url/Service.asmx?WSDL', array( 'local_cert' => 'file.cer' ));$client->__setSOAPHeaders(array($header));$param = array( 'X12Input'=>"testing", "GediPayerID"=>"52", "ResponseDataType"=>"Xml");//this leads to private key errorecho $result = $client->DoInquiryByX12Data($param,$header);\[/code\]