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). Use of the backtick operator is identical to shell_exec().
$output = `ls -al`;
The backtick operator is disabled when safe mode is enabled or shell_exec() is disabled.
Unlike some other languages, backticks have no special meaning within double-quoted strings.
See also the manual section on Program Execution functions, popen() proc_open(), and Using PHP from the commandline.