Debugging PHP using Apache Error Log

PHP runs on the server side and behaves like a function that return a value against the given arguments. A remote client may call this function and expect a specified return value and nothing else. So how do we debug this function? It must not return debugging messages since the client is never designed to handle them. We must never burden any client to handle debugging messages.

If we run PHP through Apache server then we can use the error log to keep our debugging messages. It may not be the best way to do it. But we only want to talk about this approach now.

Error Logs

The Apache error log files generally can be found in the following directory:

var/log/apache2

We issue the following command from within the directory to read the latest error messages:

# tail error.log

The tail command reads the last few lines from the error.log file and prints them on the terminal. If we need to read a specific number of lines from the end of the file then we can specify the -n option:

# tail -n 4 error.log

The command above prints the last 4 lines.

If we want to monitor the error messages in real time then we can issue the following command:

# tail -f error.log

We need to press Ctrl-C to break the monitoring.

Debugging

In PHP we can use the error_log() function to write to the Apache error.log file as follow:

error_log("My debug message");

The word error in the function name may be misleading since a debugging message may not necessarily means an error. We just have to get use to it.