Common problems.
REMOVED FUNCTIONS
Older code calling functions removed in newer versions produces a fatal error.
The error log names the file and function.
DEPRECATED WARNINGS
Not fatal, and they indicate code that will break in future.
They may also fill your log, which consumes disk.
OLD PLUGINS AND THEMES
The most common cause. Something unmaintained for years.
CUSTOM CODE
Written for an older version and never revisited.
Frequently in a theme's functions file or a custom plugin.
DATABASE DRIVERS
Very old code using removed database functions.
That usually indicates code old enough to warrant replacing.
WHAT THE SYMPTOM LOOKS LIKE
A blank page, or a 500 error, or one feature failing.
WHAT TO DO
Revert, read the log, fix or replace the component, try again.
IF THE COMPONENT IS ABANDONED
Replace it. Code requiring an unsupported PHP version is a liability.
