Category Archives: Web Development

Fixing a download link downloading the same file when it should be dynamic

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One of my clients, the Database of Pollinator Interactions allows researchers to easily search on which insects and mammals pollinate which plants. To make it simple for researchers to gather the references to papers they need to look up, their website allows downloading of search results as a CSV file.

This is powered by a little bit of AJAX, when a searcher clicks the download button, Javascript calls on a PHP script which reads the search filters out of a cookie, compiles the results into a comma separated file, and lets you download it.

However, the site had a bug (no pun intended). If you ran a search and downloaded the results, then ran a different search and downloaded the results, you got the same file of results, even if the search was completely different and the results shown on the page were correct for the search.

This turned out to be because the live server was set up to cache pages where possible, whereas my development server was not. The call to the script that made the file was on a URL that did not change, as it read what it required from a cookie. So the browser thought it was hitting the same URL each time the download button was pressed, so to help speed things up served up the file for download from its cache, rather than requesting a new one from the website.

The fix for this was quite straightforward. In the PHP script that received the call from Javascript, I added these headers at the top of the code:

header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

Having them all is probably overkill and I should go back and find which one really does the job for them.

This makes the script send headers when the browser requests the URL saying not to cache what is sent back. So the browser will request it fresh each time the URL is called.

Now, when the button is pressed, a new CSV is always requested rather than the browser providing the one it has already received. Problem solved.

Solving a WordPress update problem on Cloudways hosting

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I have several sites hosted on Cloudways, both my own and access to clients ones. I hit a weird problem where WordPress on some sites would update fine automatically or through the admin area of WordPress, but it would not on my own site.

Recently while moving a friend’s site across different parts of my hosting, I found the problem.

Cloudways gives me two levels of SSH account to access the sites on each virtual server. A ‘master’ account which can access the files on all sites and an account on each website which can only access that particular site.

For my own site I hadn’t bothered making an SSH account for it, I just used the master account to upload the files. Not having an SSH account seemed better for security, even though you can turn them off quite easily.

This meant PHP did not have enough permissions to overwrite the files when it came time for WordPress to update itself. For the Farm site, I’d given Haze a website-specific SSH account to upload the files, and that version of WordPress could update itself without issue.

So, I made an SSH account just for my site. Ah-ha, I thought, rather than have to delete the site and re-upload it with the new account, I’ll use ‘chown’ to reassign the files to the user I’ve just created. But… no dice. Something either in their setup or my commands wasn’t working.

Rather than spend even more time faffing, I deleted the site and re-uploaded it using the new details. Now WordPress can update itself with no problems. [Update:] See below for a comment from Mustaasam from Cloudways with how to solve this with a few clicks rather than deleting and re-uploading.

If you’re a Cloudways customer and are having problems with WordPress not updating itself, check how you uploaded the files in the first place. You can do this by using SSH to view the site and ‘ls -la’ to list all the files and which account owns them – if it’s your master account, try deleting the files and upload them again with your site specific SSH account.

Thanks go to Tony Crockford and Matthew Beck for helping me with WordPress and pointing me in directions that eventually lead to me working all this out.