Suppose your user name is jimgreen and you can run the following command to get a very specific CPU usage of it in real time:

top -b -n 1 -u jimgreen | awk 'NR>7 { sum += $9; } END { print sum; }'

You can even further capture the usage metric in PHP and do conditional actions:
$usage = shell_exec("top -b -n 1 -u jimgreen | awk 'NR>7 { sum += $9; } END { print sum; }'");
Now $usage contains the CPU usage of the user jimgreen.