/**
* Returns the total width in points of the string using the specified font and
* size.
*
* This is not the most efficient way to perform this calculation. I'm 
* concentrating optimization efforts on the upcoming layout manager class.
* Similar calculations exist inside the layout manager class, but widths are
* generally calculated only after determining line fragments.
*
* @param string $string
* @param Zend_Pdf_Resource_Font $font
* @param float $fontSize Font size in points
* @return float
*/
function widthForStringUsingFontSize($string, $font, $fontSize)
{
    $drawingString = iconv('', 'UTF-16BE', $string);
    $characters = array();
    for ($i = 0; $i < strlen($drawingString); $i++) {
        $characters[] = (ord($drawingString[$i++]) << 8) | ord($drawingString[$i]);
    }
    $glyphs = $font->cmap->glyphNumbersForCharacters($characters);
    $widths = $font->widthsForGlyphs($glyphs);
    $stringWidth = (array_sum($widths) / $font->getUnitsPerEm()) * $fontSize;
    return $stringWidth;
}

$font = Zend_Pdf_FontFactory::fontWithName(Zend_Pdf_Const::FONT_HELVETICA);
$stringWidth = widthForStringUsingFontSize('Hello world!', $font, 12);