Error constructing a mysql chain

I’m getting crazy. I try to do something as simple as to make a mysql chain: $my_chain = “INSERT INTO my_table SET my_field=’$my_variable'”;

When I print $my_chain from the code I get something like this: INSERT INTO my_table SET my_field=”the_variable_value

Someone can tell me what the hell happens there?. PD: The same happens using INSERT INTO (my_table) VALUE ($my_variable)