PHP验证XML格式是否正确

    function validxml(){
    	libxml_use_internal_errors(true); 
		$xmlstring = $_POST["xmlString"];//获取xml字符串
		$xml = new DOMDocument(); 
		$xml -> loadXML($xmlstring); //读取xml
		$xsdname='';
		if($_POST["xsdname"]!=""){//传递xsd文件名
			$xsdname=$_POST["xsdname"];
		}else{
			$xsdstring = $_POST["xsdString"];//否则读取xsd字符串
			$myfile = fopen("valid.xsd", "w") or die("Unable to open file!");//打开xsd文件
			fwrite($myfile, $xsdstring);//写入字符串
			fclose($myfile);//关闭
			$xsdname='./valid.xsd';//设置文件名
		}
		if (!$xml->schemaValidate($xsdname)) {//验证
			libxml_display_errors(); 
		} 
		else{ 
			echo "验证通过且格式良好"; 
		}
    }
    function libxml_display_errors() { 
	    $errors = libxml_get_errors(); 
	    $return="";
	    foreach ($errors as $error) { 
			$return.= "
"; 
			switch ($error->level) { 
				case LIBXML_ERR_WARNING:
					$return .= "Warning $error->code: "; 
					break; 
				case LIBXML_ERR_ERROR: 
					$return .= "Error $error->code: "; 
					break; 
				case LIBXML_ERR_FATAL: 
					$return .= "Fatal Error $error->code: "; 
					break; 
			} 
			$return .= trim($error->message); //获取错误
			if ($error->file) { 
				$return .= " in xml file"; 
			} 
			$return .= " on line $error->line";  //获取错误位置
	    } 
		echo $return;//返回值
		libxml_clear_errors(); 
	}