from the pstree output, php run in SuEXEC daemon mode as expected.
>APC is still restarted
because of webserver restart which because of
>Max retries has been reached, 503!
>oops! 503 Service Unavailable
now should trouble-shoot the 503 error issue. when it's resolved, APC issue will be fixed too.
regarding php 503 error, it's because of php process crash. there are many reasons a php process could crash. a simple php script won't cause 503 error.
1st, check the php extensions you've installed. see if able to disable some or all of them to reduce 503 error. please paste here the list of your current extensions(.so)