The constant must be declared before including adodb.inc.php. It is not possible to change the default metatype during a procedure.

The constant ADODB_DEFAULT_METATYPE controls the returned metatype of an unknown field type . The default value for this constant is 'N'.

This constant is useful for example in geometry data types where there is no defined metaType in ADOdb

// Connecting to a postgreSql database
DEFINE(ADODB_DEFAULT_METATYPE, 'C');
require "adodb/adodb.inc.php";
 
$db = NewADOConnection('postgres');
// connection assumed
 
$m = $db->metaType('polygon');
print $m;
// Unknown, returns: 'C'