Thinkphp 调用PHP文件实例化类 必须反斜线

<?php
namespace app\portal\service;

include "/home/wwwroot/test.test.org.cn/vendor/pdf/fpdf.php";
class PdfService
{
    public function TitleList($title, $files){
        $pdf = new \FPDF();
        $pdf->AddPage();
        $pdf->SetFont('Arial', 'B', 16);
        $pdf->Cell(0, 10, 'Hello World!', 0, 1, 'C');
        $pdf->Output('/home/wwwroot/test.test.org.cn/public/example.pdf', 'F');

    }
}

导入PHP文件 include "/home/wwwroot/test.test.org.cn/vendor/pdf/fpdf.php";

$pdf = new \FPDF(); 实例化类 必须反斜线