Use wordwrap and explode:
PHP Code:
$wrapped = wordwrap($description, 68, PHP_EOL, true);
$aLines = explode(PHP_EOL, $wrapped);
$iLines = count($aLines);
$increase = 5;
for ($i = 0, $j = 25; $i < $iLines; ++$i, $j += $increase)
{
$pdf->writeHTMLCell(139, 5, $j, 125, $aLines[$i], '', 1, 1, true, 'L', true);
}