linux - php shell_exec can't return "pidof php" on CentOS

I am using shell_exec command to get the back end processes of PHP. Like:

$dir = shell_exec("ls -la");  //returns directory listing
$date = shell_exec("date");  //returns date
$pid = shell_exec("pidof php"); //returns nothing !!!
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If I run"pidof php" from command console as root, it gives me the correct result. I'm usingPHP 5.5 andCentOS 6.5.

What additionally I need to do to get the processes?

N.B: If I run the file from command line, it gives me the correct result. But the same file when running from browser, can't return the processes.