Common PHP errors
Start with the error log and the time of the failed request. Change one thing at a time so you can tell what helped.
500 error or blank page
Find the matching entry in the PHP and web server logs. A blank page is a symptom, not a diagnosis. Check recent deployments, plugin changes and whether the server is running the expected PHP version. Keep error display off on public pages.
Allowed memory size exhausted
The log usually names the request and limit. Compare that task with memory_limit; inspect loops, plugins and data size before increasing the cap. See the settings reference.
Uploads fail
Check both upload_max_filesize and post_max_size, plus any web server, proxy or app limit. Confirm the destination is writable and has free space.
Timeout or failed upgrade
Look for the layer that stopped the request: PHP, web server, proxy, database or upstream service. After an upgrade, compare the new PHP version and extensions with your app’s requirements. If the site is unavailable, use the emergency troubleshooting path.