youtube-dl and php exec - php

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 ?

Answer 1

exec reads only stdout, so you're missing the error message on stderr, whatever that may be.
Use the following code to get stderr as well:
$url = 'http://www.youtube.com/watch?v=coq9klG41R8';
$template = '/var/www/vhosts/my.virtual.host.net/httpdocs/downloader/' .
'downloads/%(id)s.%(ext)s';
$string = ('youtube-dl ' . escapeshellarg($url) . ' -f 18 -o ' .
escapeshellarg($template));
$descriptorspec = array(
0 => array("pipe", "r"), // stdin
1 => array("pipe", "w"), // stdout
2 => array("pipe", "w"), // stderr
);
$process = proc_open($string, $descriptorspec, $pipes);
$stdout = stream_get_contents($pipes[1]);
fclose($pipes[1]);
$stderr = stream_get_contents($pipes[2]);
fclose($pipes[2]);
$ret = proc_close($process);
echo json_encode(array('status' => $ret, 'errors' => $stderr,
'url_orginal'=>$url, 'output' => $stdout,
'command' => $string));
Most likely, you don't have permission to write to that directory. To test that theory, check whether
touch /var/www/vhosts/my.virtual.host.net/httpdocs/downloader/downloads/test
works. You can use chmod and chown to change permissions.