问题已解决.原来是一个包含空格的php类.我猜这弄乱了标题.

Problem solved. Turned out to be a included php class with whitespace in it. This messed up the headers i guess.

通过创建一个新的空项目来解决，并逐步进行操作，直到出现不良类为止.

Solved by creating a new, empty project and include step by step until the bad class showed.

在Slim函数中设置标题的有效解决方案；

Working solution for setting headers inside a Slim function;

<?php

require 'vendor/autoload.php';
$app = new \Slim\Slim();

$app->get('/foo', function () use ($app) {
    $app->response->headers->set('Content-Type', "application/pdf");
    $app->response->setBody("foo");
});

$app->run();
?>

已更新:这是我用来让用户下载PDF的标题:

Updated: This is the headers I use to let a user download a PDF:

$app->response->headers->set('Content-Type', $f->type);
$app->response->headers->set('Pragma', "public");
$app->response->headers->set('Content-disposition:', 'attachment; filename=' . $f->name);
$app->response->headers->set('Content-Transfer-Encoding', 'binary');
$app->response->headers->set('Content-Length', $f->size);
$app->response->setBody($f->data);