if (!isset($my_var)) { error_log('Warning: $my_var is not defined'); }This code uses the built-in `error_log()` function in PHP to write the warning message to the error log file. The `isset()` function checks if a variable is defined and not null. If the variable is not defined, the warning message is logged. PHP provides several logging functions apart from `error_log()` such as `trigger_error()` and `syslog()`. Additionally, there are several popular logging libraries in PHP including Monolog, Log4php, and KLogger. These libraries provide advanced logging features such as logging to multiple files and outputs. Overall, developers can use PHP log warning to improve their application's reliability and ensure that any errors or issues are logged for future debugging.