Using scripts from Open Source project phpSysInfo
it is quite easy to set up a simple system-health monitoring on any webserver that can run PHP files. This feature can be monitored using IPCheck Server Monitor
The trick is that phpSysInfo shows red bars as soon as the available memory, CPU resources or disk space fall below a certain threshold. These red bars are shown using GIFs images with the word "redbar" in their filename. Using an HTTP Advanced sensor we are monitoring that the resulting HTML from the script does not include the word "redbar". As long as this word is not included in the HTML the server works fine.
Read more in our Knowledge Base: Using phpSysInfo to monitor the system health of a PHP webserver with IPCheck Server Monitor