2021'de kullandığım test kod bloğunu paylaşıyorum umarım işinize yarar;

$giris = '<kullaniciAdi>'.$this->ApiUserName.'</kullaniciAdi>
         <sifre>'.$this->ApiUserPass.'</sifre>';

$islemXml = '<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:uet="http://uetds.unetws.udhb.gov.tr/">
   <soapenv:Header/>
   <soapenv:Body>
      <uet:kimlikDogrulama>
         <!--Optional:-->
         <wsuser>
              '.$giris.'
         </wsuser>
         <!--Optional:-->
         <kimlikBilgileriInput>
            <kimlikNo>'.$tc.'</kimlikNo>
            <adi>'.$ad.'</adi>
            <soyadi>'.$soyad.'</soyadi>
         </kimlikBilgileriInput>
      </uet:kimlikDogrulama>
   </soapenv:Body>
</soapenv:Envelope>'


$headers = array(
            "Content-type: text/xml;charset=\"utf-8\"",
            "Accept: text/xml",
            "Cache-Control: no-cache",
            "Pragma: no-cache",
            "Content-length: ".strlen($islemXml),
            );

$result = UETDSCurl($islemXml,$headers);

function UETDSCurl($islemXml,$headers) {

                $ch = curl_init();
                curl_setopt($ch, CURLOPT_URL,$this->ApiUrl);
                curl_setopt($ch, CURLOPT_POST, 1); 
                curl_setopt($ch, CURLOPT_POSTFIELDS, $islemXml);
                curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
                curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
                curl_setopt($ch, CURLOPT_TIMEOUT,10);
                curl_setopt($ch, CURLOPT_USERPWD, $this->ApiUserName . ":" . $this->ApiUserPass);
                $response = curl_exec($ch);
                $httpStatusCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
                curl_close($ch);
                if($httpStatusCode==200) {
                return $response;
            } else {
                return false;
            }


}

Kodun en düz haline çevrilmişi bu mantığını anlaman için bu şekle getirdim