<?php

$dbh = new PDO('firebird:dbname=...;role=...;charset=UTF8', "...", "...") or die;

$limit = 250;
$offset = 0;

while (true) {
   $start = $offset + 1;
   $stop = $offset + $limit;

   $sql = "SELECT * FROM tablename ROWS {$start} TO {$stop}";
   $stmt = $dbh->prepare($sql);
   $stmt->execute();
   $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
   if (count($rows) < $limit) {
     return;
   }
   unset($stmt);
   $rows = null;
   $offset += $limit;
   echo "used: " . memory_get_usage(true) . "\r\n";
}
unset($dbh);

used: 10485760
used: 16777216
used: 20971520
used: 27262976
used: 33554432
used: 37748736
used: 41943040
used: 46137344
used: 52428800
used: 58720256
used: 62914560
used: 69206016
used: 75497472

CREATE DOMAIN T_MEMO AS BLOB SUB_TYPE 1 CHARACTER SET UTF8

*ptr = S->fetch_buf[colno] = erealloc(*ptr, *len+1);

*ptr = FETCH_BUF(S->fetch_buf[colno], char, *len, NULL);