The easiest way find the process of an application running on the server is to type the following command:

ps aux | grep 'apache'

This command would output a list of processes.  I can  see that the calling a /usr/sbin/apache2 is www-data. So the username of apache is www-data.

www-data  4126  0.0  0.3  32404 14296 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4132  0.0  0.1  26048  4520 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4134  0.0  0.3  32512 13964 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4135  0.0  0.1  26048  4516 ?        S    12:28   0:00 /usr/sbin/apache2 -k start
www-data  4145  0.0  0.3  31892 12964 ?        S    12:29   0:00 /usr/sbin/apache2 -k start
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Note the address /usr/sbin/apache2 -k start. This indicates the command that launched the process. The first word of each process is its username. The username for apache is www-data.

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