SQL Server 2008 stops eventually

IIS 7.5 (Windows Server 2008 R2)

PHP 5.4.14

Sqlsrv drivers 3.0.1

SQL Server 2008 R2

The database eventually stops working totally, and the only thing I can do is restart SQL Server. The first time it gave this error, after 3 hours working:

Error: 17886<c/> gravedad: 20<c/> estado: 1.,

The server will drop the connection<c/> because the client driver has sent multiple requests while the session is in single-user mode. This error occurs when a client sends a request to reset the connection while there are batches still running in the session<c/> or when the client sends a request while the session is resetting a connection. Please contact the client driver vendor.

We checked our php code on a developement machine and we couldn't find anything wrong (3 people looked for a code error).

After checking the code we tried it again, and eventually the database stopped answering again, but it didn't gave any error.

My connection string is:

$connectionInfo = array( "Database"=>$database, "UID"=>$uid, "pwd"=>$pwd, "ConnectionPooling"=>0);
$conn= sqlsrv_connect( $serverName, $connectionInfo);