PHP supports one execution operator: backticks (``).
Note that these are not single-quotes! PHP will attempt to
execute the contents of the backticks as a shell command; the
output will be returned (i.e., it won't simply be dumped to
output; it can be assigned to a variable).
注: The backtick operator is disabled when safe mode is enabled or shell_exec() is disabled.
See also escapeshellcmd(), exec(),
passthru(), popen(),
shell_exec(), and system().