Soap call to code convertion

I've been struggeling with this for way to long now, 

I'm trying to have a service call working in PHP, that does work perfectly in SoapUI....

A service called "CheckIn@work" uses a what's called X.509 authentication. this users a p12-keystore with alias and password.

The screen above is the setting I currently use for the request. The timestamp has just the (default?) value of 60 seconds.

The keystore configuration is the follwoing:

Now for the request configuration:

Endpoint: https://services-sim.socialsecurity.be/SecurityTokenService/v1

The body is (and needs to be exacly) the follwoing:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Header/>
       <soap:Body>
      	<wst:RequestSecurityToken Context="abc" xmlns:wst="http://docs.oasis-open.org/ws-sx/ws-trust/200512">
      	<wst:TokenType>http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.1#SAMLV1.1</wst:TokenType>
      	<wst:RequestType>http://docs.oasis-open.org/ws-sx/ws-trust/200512/Issue</wst:RequestType>
          <wst:Claims Dialect="http://schemas.xmlsoap.org/ws/2006/12/authorization/authclaims">
              <auth:ClaimType Uri="urn:be:smals:expeditor:number" xmlns:auth="http://schemas.xmlsoap.org/ws/2006/12/authorization">
                  <auth:Value>[AN_UNIQUE_VALUE]</auth:Value>
              </auth:ClaimType>
          </wst:Claims>
      	</wst:RequestSecurityToken>
       </soap:Body>
</soap:Envelope>

I then get a big response if the request worked. In my case, the request succeeds in SoapUI, but it does not in PHP.

The WSDL data looks like the following:

I have no idea what SoapUI does in the background of authentication with the X509 certificate.

The whole "tutorial" on how the SouapUI is being set up, can be found here https://www.socialsecurity.be/site_nl/general/helpcentre/soa/overview.htm (dutch)

I can share my PHP code, but I don't think that that is "allowed" here, so i'll leave that for now (except if someone would need or want to see it).

Hoping for someone who can explain to me how that soapUI handles this request and how I can reproduce this in another (PHP) language...

Kindest regards

Robin Rosiers