syslog()function from the PHP standard library, as well as other messages printed to the standard error stream. App Engine also records each request in the log. Each log severity level (4 for CRITICAL, 3 for ERROR, 2 for WARNING, 1 for INFO, 0 for DEBUG) has a fixed buffer size that controls the amount of log information you can access. Normally, you use logging features more at lower log levels; thus, the time window is smaller for log events at these levels.
To browse your app's logs of the last 90 days, you can also use the Logs page in the Google Cloud console.