youtube-dl and php exec

Question

I have installed youtube-dl on my CentOS 6 / Plesk 10 Dedicated server, and via SSH, everything works like a charm !

The thing is I'm trying to write a php script which takes a POST parameter in containing the URL of the video I want to copy to the server, copy it and then process it with ffmpeg.

I thought I would use the exec() function.

I can get an output if I echo the results of youtube-dl --help, but everytime I ask php to execute a command which actually does something with the video, it returns a status of '1', and outputs nothing.

Any ideas on what I'm doing wrong ?

Here is my php code:

<?php 
    $result = array();
    $status;
    $url = $_POST['src'];
    $string = 'youtube-dl "'.$url.'" -f 18 -o "/var/www/vhosts/my.virtual.host.net/httpdocs/downloader/downloads/%(id)s.%(ext)s"';
    $string2 = 'youtube-dl --help';
    exec($string, $result, $status);
    echo json_encode(array('status' => $status, 'url_orginal'=>$url, 'url' => $result));
?>

When I execute $string2, I get status: "0" and "url": [youtube-dl help text lines]

But when I execute $string, nothing happens, I get "status": "1" and nothing else, no video is downloaded. I've tried also a simulation with the "-g" parameter, and variants but as soon as youtube-dl has to fetch the video, it breaks.

Thank you in advance !

EDIT

I edited my code so it looks like this :

<?php 
    $result = array();
    $status;
    $url = $_POST['src'];
    $string = 'youtube-dl "'.$url.'" -f 18 -o "/var/www/vhosts/my.virtual.host.net/httpdocs/downloader/downloads/%(id)s.%(ext)s"';
    $string2 = 'youtube-dl --help';
    exec($string, $result, $status);
    echo json_encode(array('status' => $status, 'url_orginal'=>$url, 'url' => $result, 'command' => $string));
?>

and the result, which I didn't get yesterday now is :

command: "youtube-dl "http://www.youtube.com/watch?v=coq9klG41R8" -f 18 -o "/var/www/vhosts/my.virtual.host.net/httpdocs/downloader/downloads/%(id)s.%(ext)s""
status: 1
url: 
    0: "[youtube] Setting language"
    1: "[youtube] coq9klG41R8: Downloading video info webpage"
    2: "[youtube] coq9klG41R8: Extracting video information"
url_orginal: "http://www.youtube.com/watch?v=coq9klG41R8"

Which is weird, considering a) that yesterday I got an empty url[], and that b) even if now I get what apparently looks like a normal youtube-dl return, it only contains the 3 first lines, and I cannot see any video file in the specified path... Any ideas ?