PHP exec mocp command

Question

I want to play music via PHP on my Raspberry Pi with Mocp (music on console). This command

mocp -S

works just fine in the console (it starts the server of mocp) but in PHP's exec function it doesn't work:

<?php
    $ret = exec("mocp -S", $out, $err);
    print_r($ret);
    print_r($out);
    print_r($err);
?>

It just returns:

Array ( ) 2

According to this list return_var code 2 means

Misuse of shell builtins (according to Bash documentation)

But what exactly does this mean? The command mocp --help works in both console and PHP exec. How can I run mocp with every parameter in PHP?

EDIT:
I just logged in as "www-data" (the apache user) and tried to run mocp commands. I got this return:

FATAL_ERROR: Can't create directory /var/www/.moc

I copied the .moc folder of Pi to www-data (with user "pi"):

sudo cp /home/pi/.moc/ /var/www/.moc

Now my script from above gives me:

Running the server...Array ( [0] => Running the server... [1] => Running the server... ) 2

But it still does not work. In the console of www-data mocp -S now gives me

FATAL_ERROR: Can't bind() to the socket

EDIT 2:
I changed the permsions on /var/www/.moc. If I try mocp -S, I get this

Running the server...
Trying JACK...
Trying ALSA...
ALSA lib confmisc.c:768:(parse_card) cannot find card '0'
ALSA lib conf.c:4241:(_snd_config_evaluate) function snd_func_card_driver returned error: Datei oder Verzeichnis nicht gefunden
ALSA lib confmisc.c:392:(snd_func_concat) error evaluating strings
ALSA lib conf.c:4241:(_snd_config_evaluate) function snd_func_concat returned error: Datei oder Verzeichnis nicht gefunden
ALSA lib confmisc.c:1251:(snd_func_refer) error evaluating name
ALSA lib conf.c:4241:(_snd_config_evaluate) function snd_func_refer returned error: Datei oder Verzeichnis nicht gefunden
ALSA lib conf.c:4720:(snd_config_expand) Evaluate error: Datei oder Verzeichnis nicht gefunden
ALSA lib pcm.c:2217:(snd_pcm_open_noupdate) Unknown PCM default
Trying OSS...
FATAL_ERROR: No valid sound driver!
FATAL_ERROR: Server exited!

Answer 1

Seems like www-data user doesn't have right access permissions to use your audio device. I belive executing this should help:

sudo usermod -a -G audio www-data

Answer 2

My full solution of the problem:
I started with a new installation of raspbian and ran those comands:
sudo raspi-config ->Expand Filesystem ->Finish, reboot sudo apt-get update sudo apt-get upgrade sudo apt-get install apache2 php5 libapache2-mod-php5 sudo groupadd www-data sudo usermod -g www-data www-data sudo apt-get install moc sudo usermod -a -G audio www-data mocp -S sudo cp -R /home/pi/.moc /var/www/.moc sudo chown www-data /var/www/.moc/ sudo chmod -R a+rwxst .moc/
At the end user "www-data" is able to run mocp.