Connection Pooling (Microsoft Drivers for PHP for SQL Server)

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The following are important points to note about connection pooling in the Microsoft Drivers for PHP for SQL Server:

  • The Microsoft Drivers for PHP for SQL Server uses ODBC connection pooling.

  • By default, connection pooling is enabled in Windows. In Linux and macOS, connections are pooled only if connection pooling is enabled for ODBC (see Enabling/Disabling connection pooling). When connection pooling is enabled and you connect to a server, the driver attempts to use a pooled connection before it creates a new one. If an equivalent connection is not found in the pool, a new connection is created and added to the pool. The driver determines whether connections are equivalent based on a comparison of connection strings.

  • When a connection from the pool is used, the connection state is reset (Windows only).

  • Closing the connection returns the connection to the pool.

For more information about connection pooling, see Driver Manager Connection Pooling.

Enabling/Disabling connection pooling


You can force the driver to create a new connection (instead of looking for an equivalent connection in the connection pool) by setting the value of the ConnectionPooling attribute in the connection string to false (or 0).

If the ConnectionPooling attribute is omitted from the connection string or if it is set to true (or 1), the driver only creates a new connection if an equivalent connection does not exist in the connection pool.


Multiple Active Result Sets (MARS) is enabled by default. When both MARS and pooling are in use, in order for MARS to work correctly, the driver requires a longer time to reset the connection on the first query, thus ignoring any query timeout specified. However, the query timeout setting will take effect in the subsequent queries.

If necessary, please check How to: Disable Multiple Active Resultsets (MARS). For information about other connection attributes, see Connection Options.

Linux and macOS

The ConnectionPooling attribute cannot be used to enable/disable connection pooling.

Connection pooling can be enabled/disabled by editing the odbcinst.ini configuration file. The driver should be reloaded for the changes to take effect.

Setting Pooling to Yes and a positive CPTimeout value in the odbcinst.ini file enables connection pooling.


[ODBC Driver 17 for SQL Server]
CPTimeout=<int value>

Minimally, the odbcinst.ini file should look something like this example:


[ODBC Driver 17 for SQL Server]
Description=Microsoft ODBC Driver 17 for SQL Server

Setting Pooling to No in the odbcinst.ini file forces the driver to create a new connection.



  • In Linux or macOS, connection pooling is not recommended with unixODBC < 2.3.7. All connections will be pooled if pooling is enabled in the odbcinst.ini file, which means the ConnectionPooling connection option has no effect. To disable pooling, set Pooling=No in the odbcinst.ini file and reload the drivers.
    • unixODBC <= 2.3.4 (Linux and macOS) might not return proper diagnostic information, such as error messages, warnings and informative messages
    • for this reason, SQLSRV and PDO_SQLSRV drivers might not be able to properly fetch long data (such as xml, binary) as strings. Long data can be fetched as streams as a workaround. See the example below for SQLSRV.
$connectionInfo = array("Database"=>"test", "UID"=>"username", "PWD"=>"password");

$conn1 = sqlsrv_connect("servername", $connectionInfo);

$longSample = str_repeat("a", 8500);
$xml1 = 

// Create table and insert xml string into it
sqlsrv_query($conn1, "CREATE TABLE xml_table (field xml)");
sqlsrv_query($conn1, "INSERT into xml_table values ('$xml1')");

// retrieve the inserted xml
$column1 = getColumn($conn1);

// return the connection to the pool

// This connection is from the pool
$conn2 = sqlsrv_connect("servername", $connectionInfo);
$column2 = getColumn($conn2);

sqlsrv_query($conn2, "DROP TABLE xml_table");

function getColumn($conn)
    $tsql = "SELECT * from xml_table";
    $stmt = sqlsrv_query($conn, $tsql);
    // This might fail in Linux and macOS
    // $column = sqlsrv_get_field($stmt, 0, SQLSRV_PHPTYPE_STRING(SQLSRV_ENC_CHAR));
    // The workaround is to fetch it as a stream
    $column = sqlsrv_get_field($stmt, 0, SQLSRV_PHPTYPE_STREAM(SQLSRV_ENC_CHAR));
    return ($column);

See Also

How to: Connect Using Windows Authentication

How to: Connect Using SQL Server Authentication