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This is a global variable that determines how arrays are retrieved by recordsets. The recordset saves this value on creation (eg. in Execute or SelectLimit), and any subsequent changes to $ADODB_FETCH_MODE have no affect on existing recordsets, only on recordsets created in the future.
$ADODB_FETCH_MODE = ADODB_FETCH_NUM;
For more information on the constants used and for more granular control of this setting, see setFetchMode()