expect_popen php doesn't work

Question

I'm trying to connect my CentOS server with my router Juniper using php. I'm using a library called netconf-php. My code is:

 <?php
    include ('netconf/Device.php') ;
   //creating a new device and establishing NETCONF session
   $d= new Device("x.y.z.m", "user", "passw");
   $d->connect();
  echo 'connected to device';

 //closing device
 $d->close();
 echo "device closed";
?>

But in the connect function it stop. I have search in the function connect in the device.php and the problem is with the function $this->stream = expect_popen("ssh -o ConnectTimeout=$this->connectTimeout $this->userName@$this->hostName -p $this->port -s netconf");. The console doesn't return anything.

I have installed all the libraries that appear in the documentation.

Anybody knows what could be the problem??

Thanks in advance