How do I set a userAgent header string to a phpunit test in Laravel 5.3?

I've not done it, but try calling this method before you call visit().

    /**
     * Define a set of server variables to be sent with the requests.
     *
     * @param  array  $server
     * @return $this
     */
    protected function withServerVariables(array $server)
    {
        $this->serverVariables = $server;
        return $this;
    }

Pass in an array like this:

[
  'HTTP_USER_AGENT' => 'User agent string here'
]

or if you want to set it as the header key, user-agent, instead of the server variable key, you'd do something like this:

$headers = [
  'user-agent' => 'user agent value here'
];

$this->withServerVariables($this->transformHeadersToServerVars($headers));
// then visit and do whatever else.

@willvincent - thanks for the reply, but this didn't work in my case.