A handy tool for checking weird output is the od command. If you pipe your output to od -c you will see all the non-printing characters. When I did this with your server (having added a print of what came in to $buf from the socket) I saw the following.

0000000   L   i   s   t   e   n   i   n   g       f   o   r       c   
+o
0000020   n   n   e   c   t   i   o   n   s       o   n       p   o   
+r
0000040   t       8   8   8   8  \n   b   u   f       =       "   C   
+S
0000060   C  \r   "  \n
0000064
[download]

Applying the fix oshalla recommended solved the problem.

The od command is usually installed on *nix systems and is available for Windows.

I note also that you are interpolating whatever text you receive from the socket into your SQL statement without any validation. Consider what will happen if a nasty fragment of SQL was entered rather than a group name. You might lose data.

A good habit is to always check your data from external sources to make sure it is valid. And you might use place holders in your SQL statement rather than interpolating the group name into the string to further reduce the risk.