Spaces in a PHP file. Unexpected output

Recently, while working on a WordPress plugin, I encountered a confusing error that took me hours to debug. So here’s a post that might be obvious to PHP developers, but for those of you like me, that only touch PHP out of necessity, hopefully it saves you some time.

The problem I encountered manifested in two ways.

Manifestation 1

While working on my plugin, I started to see a warning that looked like the below. You’ll probably only see this notification, though, if you have your WordPress debug settings turned on or a plugin that provides a PHP error log. I use PHP Error Log Monitor.

The plugin generated 1 character of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

Manifestation 2

Other than the error message, everything worked fine in the plugin and in WordPress as far as I could tell, until week’s later. At some point, I logged out of my site and saw a blank white screen. No “you’ve logged out message”. No options to log back in. And the admin screen for logging in had gone blank too. This made it impossible to log in again unless I first deleted the plugin via FTP.

To make it worse, this error only seemed to occur when I uploading to my web hosting (It didn’t cause any issues at all in my local environment).

The cause

The cause of this error, though hard for a novice in PHP to find, was very simple. I had blank lines before the first line of PHP in my PHP file. It can also occur from having erroneous spaces there.

Don’t have any leading new lines or spaces as the first characters in your primary php file.

I can only assume that this occurred differently on my local machine compared to the hosting provider because of PHP versioning difference. So while I didn’t dig into this any further, here are the PHP version details of each—Incase it helps anyone in the future. Though it could have been caused by something else.

My staging site on my hosting space.

PHP version: 8.1.16
The error occurred in this setup

My local development setup (Run by Local)

Run by Local
PHP version: 8.1.9
The error didn’t occur in this setup.

Both setups were running WordPress 6.2.2

That’s it!

I’d love to know if any of these articles helped you.
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