For those who are finding that using the $cwd and $env options cause proc_open to fail (windows). You will need to pass all other server environment variables; 

$descriptorSpec = array(
       0 => array("pipe", "r"),
       1 => array("pipe", "w"),     
    );

proc_open(
        "C:\\Windows\\System32\\PING.exe localhost, 
        $descriptorSpec , 
        $pipes, 
        "C:\\Windows\\System32", 
        array($_SERVER)
);