SSH How to scrutinise what a PID is doing?

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http://stackoverflow.com – I have a rogue Apache process running on a Centos 6 Linux server, which is running up to 55% CPU and wondered how I can scrutinise exactly what function(s) it is performing? From the 'top' command I have its process ID, but how can I drill in to what it's up to? Thank you (HowTos)