How can you log errors to a file in PHP?

You can log errors to a file using the `error_log()` function. This function takes a message as an argument and writes it to the specified log file (or the default PHP error log if no file is specified). Within a custom error handler, you can format the error message and log it along with the error level, file, and line number for detailed debugging information. Ensure the web server has write permissions to the log file.