PHP/SOAP/XML根元素前面的文档中的标记必须格式良好

我正在尝试与支付网关（FirstData）交互。到目前为止，我已经创建了cURL请求，并最终实现了它。现在，我可以在SOAP中生成包装的XML了。我收到一个错误，上面写着：

输入SOAP请求的格式不正确。例外情况是org.xml.sax.saxpasseeption：根元素前面的文档中的标记必须格式良好

下面是我提供给它的xml：

$this->xml = "<?xml version=\"1.0\"?>";
$this->xml = "<SOAP-ENV:Envelope xmlns:SOAP-ENV=\"http://schemas.xmlsoap.org/soap/envelope/\">";
$this->xml = "<SOAP-ENV:Header />";
$this->xml = "<SOAP-ENV:Body>";
$this->xml = "<fdggwsapi:FDGGWSApiOrderRequest xmlns:fdggwsapi=\“http://secure.linkpt.net/fdggwsapi/schemas_us/fdggwsapi\">";
$this->xml = "<v1:Transaction xmlns:v1=\"http://secure.linkpt.net/fdggwsapi/schemas_us/v1\">";
$this->xml = "<v1:CreditCardTxType>";
$this->xml = "<v1:Type>sale</v1:Type>";
$this->xml = "</v1:CreditCardTxType>";
$this->xml = "<v1:CreditCardData>";
$this->xml = "<v1:CardNumber>4012000033330026</v1:CardNumber>";
$this->xml = "<v1:ExpMonth>12</v1:ExpMonth>";
$this->xml = "<v1:ExpYear>12</v1:ExpYear>";
$this->xml = "</v1:CreditCardData>";
$this->xml = "<v1:Payment>";
$this->xml = "<v1:ChargeTotal>12</v1:ChargeTotal>";
$this->xml = "</v1:Payment>";
$this->xml = "</v1:Transaction>";
$this->xml = "</fdggwsapi:FDGGWSApiOrderRequest>";
$this->xml = "</SOAP-ENV:Body>";
$this->xml = "</SOAP-ENV:Envelope>";

return $this->xml;

*genXML（）包含上面列出的XML的XML消息

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欢迎提供任何信息

你的意思是

$this->xml.=“…”

？否则每次都会覆盖字符串，对吗

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