Not downloading when executed via PHP

[evolucja]

When I run test.php:

$cmd = "youtube-dl https://www.youtube.com/watch?v=zyqRNn1ouMU -f 137+bestaudio -    o '/var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.mp4' -v";
exec($cmd ,$op);
print_r($op);

i'm getting:

Array
(
    [0] => [youtube] Setting language
    [1] => [youtube] zyqRNn1ouMU: Downloading webpage
    [2] => [youtube] zyqRNn1ouMU: Downloading video info webpage
[3] => [youtube] zyqRNn1ouMU: Extracting video information
)
0

However when I run the same command via SSH - it works perfectly and downloads the files, then merges it. What can be wrong about it?

This is the output from test.php after running the same command via SSH:

Array
(
    [0] => [youtube] Setting language
    [1] => [youtube] zyqRNn1ouMU: Downloading webpage
    [2] => [youtube] zyqRNn1ouMU: Downloading video info webpage
    [3] => [youtube] zyqRNn1ouMU: Extracting video information
    [4] => [download] /var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.f137.mp4 has already been downloaded
    [5] => [download] /var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.f140.mp4 has already been downloaded
    [6] => [ffmpeg] Merging formats into "/var/zpanel/hostdata/zadmin/public_html/zgrajto_pl/d/test.mp4"
    [7] => [debug] ffmpeg command line: avconv -y -i /var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.f137.mp4 -i /var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.f140.mp4 -c copy /var/zpanel/hostdata/zadmin/public_html/evolucjaeu/d/test.mp4
)

@edit
Okay, solved it - the problem was the folder writing permission.