WampServer

<?PHP $dir = 'explorer.exe c:\\'; exec($dir);?>

there is a system process explorer,but it will not appear on the screen
then im wait 30 sec,and displays an error:
"Fatal error: Maximum execution time of 30 seconds exceeded in D:\wamp\www\work\open.php on line 10 Call Stack #TimeMemoryFunctionLocation 10.0002670072{main}( )..\open.php:0"

Try using the output and return val params for exec, as described in the manual.
That should help with debugging.