This is a MySQL setting not php, so you would either need to set this in your my.ini or other MySQL config file, or you can try setting this in the code directly.
On your options_Add.php page try adding in this code just after line 45:
php:
<?php
mysql_select_db($database_PowerStoreConnection, $PowerStoreConnection);
$set_query = "SET SESSION SQL_BIG_SELECTS=1";
$rsSet = mysql_query($set_query, $PowerStoreConnection) or die(mysql_error());
?>
This should execute the query before any of the others run and set this value for you for the duration of the session. Please give this a try and let us know how it works.
