PHP code exec ("Kill $pid" $op, $returns) cannot kill the process under the Linux virtual machine

PHP code exec ("Kill $pid", $op, $returns) cannot kill a process under a Linux virtual machine
Title, I write PHP code under Linux exec ("Kill $pid", $op, $returns), through the PID kill process, after execution, the return is always "1", that is, the execution is unsuccessful, the process ID is still in after execution. The file that executed the kill call I have given the maximum permission. Online there is said to be php.ini configuration problem what, but found a day has not found any worthy of reference, ask the great God to give some advice AH!!!!!

------to solve the idea----------------------
No permissions.
Set Apache permissions to root and see.

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