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Uploads, multipart/form-data & UploadedFile

This page uses a Symfony form. And we will learn how to add a file upload field to a form object. But... let's start simpler - with a good old-fashioned HTML form.

The controller behind this page live at src/Controller/ArticleAdminController.php, and we're on the edit() action. Create a totally new, temporary endpoint: public function temporaryUploadAction(). We're going to create an HTML form in our template, put an input file field inside, and make it submit to this action. Add the @Route() with, how about, /admin/upload/test and name="upload_test". But... don't do anything else yet.

... lines 1 - 14
class ArticleAdminController extends BaseController
{
... lines 17 - 69
/**
* @Route("/admin/upload/test", name="upload_test")
*/
public function temporaryUploadAction(Request $request)
{
... line 75
}
... lines 77 - 113
}

Copy the route name, then open the template for the edit page: templates/article_admin/edit.html.twig. The Symfony form lives inside the _form.html.twig template. So, above that form tag, add a new form tag, with method="POST" and action="" set to {{ path('upload_test') }}. Inside, we only need one thing <input type="file">. We need to give this a name so we can reference it on the server: how about name="image".

Finally, add <button type="submit"> and I'll add some classes so that this isn't the ugliest button ever. Say: Upload!

... lines 1 - 5
<form method="POST" action="{{ path('upload_test') }}">
<input type="file" name="image">
<button type="submit" class="btn btn-primary">Upload!</button>
</form>
<hr>
{{ include('article_admin/_form.html.twig', {
button_text: 'Update!'
}) }}
... lines 17 - 24

That's it! The simplest possible file upload setup: one field, one button.

Fetching the File in the Controller

In some ways, uploading a file is really no different than any other form field: you're always just sending data to the server where each data has a key equal to its name attribute. So, the same as any form, to read the submitted data, we'll need the request object. Add a new argument with a Request type-hint - the one from HttpFoundation - $request. Then say: dd() - that's dump & die - $request->files->get('image'). I'm using image because that's the name attribute used on the field.

... lines 1 - 10
use Symfony\Component\HttpFoundation\Request;
... lines 12 - 72
public function temporaryUploadAction(Request $request)
{
dd($request->files->get('image'));
}
... lines 77 - 115

Cool! What do you think this will dump out? A string filename? An array? An object? Let's find out! Choose a file - I'll go into my I <3 Space directory, and select the astronaut photo! Upload!

multipart/form-data

Oh! It's... null!? I did not see that coming. If you're ever uploading a file and it's totally not working, you've probably made the same mistake I just did. Go back to the template and add an attribute to the form enctype="multipart/form-data".

... lines 1 - 5
<form method="POST" action="{{ path('upload_test') }}" enctype="multipart/form-data">
... lines 7 - 9
</form>
... lines 11 - 24

Yep! Mysteriously, you never need this on your forms... until you have a file upload field. It basically tells your browser to send the data in a different format. We're going to see exactly what this means soon cause we are crushing the magic behind uploads.

Fortunately, PHP understand this format and this format supports file uploads. Refresh the form so the new attribute is rendered. Let's choose the astronaut again. And before hitting Upload, open up your developer tools and go to the Network tab: I want to see what this request looks like. Hit upload!

Nice! This time we get an UploadedFile object full of useful data.

But before we dive into that, look down at the network tools and find the POST request we just made. If you look at the request headers... here it is: our browser sent a Content-Type: multipart/form-data header. This is because of the enctype attribute. It also added this weird boundary=----WebkitFormBoundary, blah, blah, blah thing.

Ok: this stuff is super-nerdy-cool. Normally, when you do not have that enctype attribute, when you submit a form, all of the data is sent in the body of the request in a big string full of what looks like query parameters. That's kind of invisible to us, because PHP parses all of that and makes the data available.

But when you add the multipart/form-data attribute, it tells our browser to send the data in a different format. It's actually kind of hard to see what the body of these requests look like - Chrome hides it. No worries! Through the magic of TV... boom! This is what the body of that request looks like.

Weird, right! Each field is separated by this mysterious WebkitFormBoundary thing... which is the string that we saw in the Content-Type header! Our form only has one field, but if we had multiple, this separator would be between every field. Our browsers invents this string, separates each piece of data with it, then sends this separator up with the request so that the server knows how to parse everything.

Why is this cool? Because we can now send up multiple pieces of information about our name="image" field, like the original filename on our system and what type of file it is... which, by the way, can be totally faked by the user. More on that later. After all that, we've got the data itself!

If you look all the way at the bottom, it has another WebKitFormBoundary line. If there were more fields on this form, you'd see their data below - all separated by another "boundary".

So... that's it! It literally tells our browser to send the data in a different format - and PHP understands both formats just fine. We need this format when doing file uploads because a file upload is more than just its contents: we also want to send some metadata. And also, due to how the data is encoded, if you were able to send binary data on a normal request - without the multipart/form-data encoding - it would increase the amount of data you need to upload by as much as three times! Not great for uploads!

The UploadedFile Object

Once the data arrives at the server, PHP automatically reads in the file and saves it to a temporary location on your server. Symfony then takes all of these details and puts it into a nice, neat UploadedFile object. You can see the originalName: astronaut.jpeg, the mimeType and - importantly - the location on the filesystem where the file is temporarily stored.

If we do nothing with that file, PHP will automatically delete it at the end of the request. So... our job is clear! We need to move that into a final location and... do a bunch of other things, like make sure it has a unique filename and the correct file extension. Let's handle that next.

Leave a comment!

  • 2019-02-19 weaverryan

    Hey @CuriousPete!

    I like that you were interested enough to ask that question :). I used Charles - the debugging proxy - it catches all traffic and you can see the raw body :).

    Cheers!

  • 2019-02-19 CouriousPete

    Thx Ryan! Nice stuff as always!! I really appreciate that! I have one question out of curiosity, what kind of tool you have used to dump multipart/form-data request?