Topic
Fixes for the errors that take a WordPress site down.
Troubleshooting
The upload reached WordPress and the destination folder exists, but PHP was not allowed to put the file in it. The folder the message names is the one to inspect, and an existing folder with the wrong owner is the usual cause.
Troubleshooting
PHP started writing the upload to its temporary folder and ran out of room partway through. The disk that is full is usually the temporary partition, not the one your uploads folder sits on, which is why freeing space in wp-content often changes nothing.
Troubleshooting
The message is PHP's, not WordPress's: PHP could not create the temporary file an upload is written to before any WordPress code runs. The fix lives in PHP's configuration, which is why WP_TEMP_DIR does nothing for it.
Troubleshooting
A high TTFB is either a cache that is not being hit or a PHP request that does too much work. Measure which one you have before changing anything, because the fixes do not overlap.
Troubleshooting
A slow WordPress site is slow in one layer: the server, the page, one heavy template, the admin or logged-in sessions. Two measurements tell you which, and the fix follows from that.
Troubleshooting
The warning names the file and line that produced output too early, in the phrase in parentheses. Everything else in the message is the victim. Fix the file it names, and the login redirects, feeds and cookie errors that came with it stop together.
Troubleshooting
WordPress needs the mysqli extension and the PHP that is serving the site does not have it loaded. The database is fine. The extension is one checkbox in a hosting panel, one package on a server, or one line in a Docker image, and the error appears right after a PHP version change.
Troubleshooting
The fatal names a file and a line, and that line is where PHP was when the clock ran out — not necessarily what was slow. Raising the limit buys time; reading the trace tells you what was spending it.
Troubleshooting
The file WordPress was asked to activate exists, but nothing in its first 8 KB says Plugin Name. That narrows it to a damaged header or an unreadable file; a missing file, a nested zip and a stray folder each produce a different message, or none.
Troubleshooting
The editor asked the REST API to save the post and got a refusal, a redirect or a non-JSON page back. The second sentence of the message tells you which, and the browser's network panel shows the actual response in under a minute.
Troubleshooting
The message ends with a question — is its parent directory writable by the server? — and the answer is usually no, but for one of two reasons: ownership on the folder, or a stale upload path from a previous server still stored in the database.
Troubleshooting
The browser is almost never the problem. WordPress sets a test cookie on the login form and looks for it on submit, and anything that changes the domain between those two requests — a URL mismatch, a cache, a proxy — makes the cookie vanish.