How to deliver a generated pdf to the browser from a plugin

I’m quite sure you can send a file “download” as a data stream provided the proper headers are sent first, in particular the proper MIME type as the Content-Type: header. I’m unsure of the exact details since I’ve not actually done this myself, I just pick up odd little bits of knowledge like this from time to time. Some more tidbits can be found at https://php.net/manual/en/function.header.php.

I’ve heard that successfully doing this in a manner compatible with all browsers is tricky to get right. Good luck!

Thanks. Sending the right headers etc is actually easy outside WordPress – FPDF handles that, and it works well. The issue is to stop WordPress sending anything else (eg the normal webpage headers).

Overnight I’ve wondered whether my link could be something like
<a href="javascript:myJsFunction();">
where the javascript function calls the WP ajax mechanism which generates a pdf file in a unique folder (and deletes old folders), then returns the url of the generated file.

I need unique folders because I want the pdf files to have a sensible name, rather than a unique name.

Anyone any experience of this? Will the browser respond to the href that the js function returns in all circumstances/with all browsers?

Thanks

Does your PDF generating file need any info from WordPress?

If not I’d use that as a stand-alone script somewhere on the site, and link to it from your WordPress site (not every request has to run through WordPress).

if it does, then you can do what bcworkz and output the PDF file through the WordPress system. Just output the right download and file type headers, then output the files contents from FPDF, and die(). I’ve done that a few times in the past, and it’s entirely possible.

Yes, I’m afraid the pdf is generated from custom tables in the WP database. I guess I could try to access the database from outside WP but that would be a nightmare.

There is something I have failed to underdstand here, and it may well be obvious:

output the PDF file through the WordPress system. Just output the right download and file type headers, then output the files contents from FPDF, and die()

Exactly how can this be done? I’m expecting to use <a href="???"> or something similar; how can I get WordPress to output the right headers, and not output headers itself? Are there hooks to intercept all of WordPress’ normal page output?

Thanks again!

[It looks as if I can place a generated file into a subfolder of the plugin folder on the server and it’ll be publicly available, so that is one hurdle out of the way. Luckily the data isn’t very private, although it’s only available to logged in members.]

Accessing DB tables outside of WP isn’t the nightmare you fear, unless there’s some odd quirk of your installation I’m unaware of. Or is it some horrendously complex SQL that is required that you’re trying to avoid? The DB can be accessed using PHP’s mysqli class. You do need some minor SQL skills, which is easily learned if the query is not too convoluted.

Outputting files for download is not exactly a walk in the park, you’ll need to setup a CRON job to regularly delete old files. The folder for these files does not need to be with the plugin. I’m inclined to create a dedicated folder in uploads myself. Yeah, it’s for downloads, so what? It doesn’t even need to be in the WP folder structure, anywhere publicly accessible will work.

Finally, creating WP code that sends headers and outputs a data stream is quite feasible. The only gotcha is that to send headers, no other output can occur beforehand. It’s not difficult to bypass the usual WP page output to avoid this situation.

Send a post or get request to wp-admin/admin-post.php. The request must have an ‘action’ parameter along with whatever other data your script will need, like a post ID or something. Also consider using a nonce scheme to validate the request. The action value is used to construct a do_action() call. Your plugin hooks this action, verifies the nonce, sends headers and generates output.

Have a look at the source for admin-post.php for details on how this works, it’s fairly straight forward.

Brilliant – thank you.

Perhaps I’m a little too cautious about accessing the WP database in a separate program – one worry being to acquire the password (yes, I know it’s in config.php or somewhere but that’s presumably not readable by another progam so it needs human intervention – less than great here as I’m not the human who’d have to do that). But it’s not out of the question, I agree. I’d not need CRON in fact, as any new request for a pdf could check for old files and delete them. So yes, this is one possible solution.

Another idea I’ve come up with is to have a <a href='xyz/file.pdf' onclick='myfunction()'>construct where I write the pdf file url, but only construct the file itself on the click. That would probably work too, if the function returned true. My previous idea to make a javascript function return a pdf file url to href (<a href="javascript:myfunction()">) doesn’t work; there is probably no specification of what should happen (I’ve waded through the HTML5 spec), and 2 browsers I tried simply return the text output of the javascript function to a new page, even converting boolean values to text.

But the one I really like is to call wp-admin/admin-post.php. I’ll try that first.

Thanks very much – very helpful.

Yes, admin-post.php can be very useful. It’s curious that it’s not promoted more in docs and various how-tos that are around.

Using javascript can work, you would end up with some sort of AJAX scheme, which in WP is somewhat similar to using admin-post.php. Once again it’s a matter of responding with the proper headers.

There’s no problem maintaining DB user and password in an external PHP app (other than it now exists in two places). Unless the server is horribly mis-configured, there is no way for users to get to the PHP code containing the password. It is no less secure than it being in wp-config.php. Don’t let that be what’s stopping you from an external app approach. I am personally leaning towards this approach only because initializing the WP environment involves a lot of overhead. An external app is bound to be more efficient.

But if you have other needs from WP besides DB access, then admin-post.php would be my next choice. I’m not that keen on a javascript approach just because it seems needlessly more complicated getting client side script involved.

All sorted – thanks very much for your help!