src/Soap/Client.php
public function setContentType($contentType = null)
    {
        if ($contentType !== null && !is_string($contentType)) {
            throw new Exception('Content-type value must be a valid string or null to use Soap verion defaults.');
        }
        $this->contentType = $contentType;
        return $this;
    }
public function setContentType($contentType = null)
    {
        if ($contentType !== null && !is_string($contentType)) {
            throw new Exception('Content-type value must be a valid string or null to use Soap verion defaults.');
        }
        $this->contentType = $contentType;
        return $this;
    }
Sets the value of the contentType variable used in the content-type
request header. Suggested values: text/xml; application/soap+xml;
type/application-xml

@param string $contentType
@return $this
@throws Exception