may help someone...

<?php
    /**
     * Split the given text into rows fitting the given maxWidth
     *
     * @param unknown_type $draw
     * @param unknown_type $text
     * @param unknown_type $maxWidth
     * @return array
     */
    private function getTextRows($draw, $text, $maxWidth)
    {        
        $words = explode(" ", $text);
        
        $lines = array();
        $i=0;
        while ($i < count($words)) 
        {//as long as there are words

            $line = "";
            do
            {//append words to line until the fit in size
                if($line != ""){
                    $line .= " ";
                }
                $line .= $words[$i];
                
                
                $i++;
                if(($i) == count($words)){
                    break;//last word -> break
                }
                
                //messure size of line + next word
                $linePreview = $line." ".$words[$i];
                $metrics = $this->canvas->queryFontMetrics($draw, $linePreview);
                //echo $line."($i)".$metrics["textWidth"].":".$maxWidth."<br>";
                
            }while($metrics["textWidth"] <= $maxWidth);
            
            //echo "<hr>".$line."<br>";
            $lines[] = $line;
        }
        
        //var_export($lines);
        return $lines;
    }
?>

Here's how to create a header image and write it to file.  This took me a while to figure out.  I hope this helps.

<?php

/* Text to write */
$text = "Hello World!";

/* Create Imagick objects */
$image = new Imagick();
$draw = new ImagickDraw();
$color = new ImagickPixel('#000000');
$background = new ImagickPixel('none'); // Transparent

/* Font properties */
$draw->setFont('Arial');
$draw->setFontSize(50);
$draw->setFillColor($color);
$draw->setStrokeAntialias(true);
$draw->setTextAntialias(true);

/* Get font metrics */
$metrics = $image->queryFontMetrics($draw, $text);

/* Create text */
$draw->annotation(0, $metrics['ascender'], $text);

/* Create image */
$image->newImage($metrics['textWidth'], $metrics['textHeight'], $background);
$image->setImageFormat('png');
$image->drawImage($draw);

/* Save image */
file_put_contents('/path/to/file.png', $image);
?>

[As mentioned in another comment] To correctly position text in the vertical plane when using `annotation()`, you can use the value of the `"ascender"` element returned by `Imagick::queryFontMetrics()`.

So, to effectively make your text touch [0, 0], for example, you could do something like the following.

<?php
$textDraw = new ImagickDraw();
$textDraw->setFont($fontPathname);
$textDraw->setFontSize($fontSize);
$textDraw->setFillColor(new ImagickPixel($color));

$imagick= new Imagick();
$textMetrics = $imagick->queryFontMetrics($textDraw, 'Hello, World!');

$textDraw->annotation(
    0,
    $textMetrics['ascender'],  // <-- This is the important bit :-)
    'Hello, World!'
);
?>

Hope that helps clarify.  The other comment was a blessing for me.

You can use this method to break your text so that it'll fit a certain $maxWidth.

<?php
/**
 * @param string $text
 * @param int $maxWidth
 */
protected function _fitText($text, $maxWidth)
{
    $im = new Imagick();
    $im->newImage($this->_width, $this->_height, "none");

    $lines = explode(PHP_EOL, trim($text));
    $DEBUG_LOOP = 0;

    for ($k = 0; $k < count($lines); ++$k) {
        do {
            $drawText = new ImagickDraw();
            // set your font settings like size, family, .. here
            $metrics  = $im->queryFontMetrics($drawText, $lines[$k]);
            $fits     = $metrics["textWidth"] <= $maxWidth;

            if ($fits) {
                break;
            }

            $pos = mb_strrpos($lines[$k], " ");
            if ($pos === false) {
                throw new RuntimeException("can not make it fit");
            }
            if (!isset($lines[$k + 1])) {
                $lines[$k + 1] = null;
            }
            $lines[$k + 1] = trim(mb_substr($lines[$k], $pos + 1) . " " . $lines[$k + 1]);
            $lines[$k]     = trim(mb_substr($lines[$k], 0, $pos));

            if (++$DEBUG_LOOP >= 200) {
                throw new RuntimeException("infinite loop");
            }
        } while (!$fits);
    }
    $text     = implode(PHP_EOL, $lines);
    $drawText = new ImagickDraw();
    // set your font settings like size, family, .. here again!
    $metrics  = $im->queryFontMetrics($drawText, $text);
    $metrics["text"] = $text;
    assert('$metrics["textWidth"] <= $maxWidth');
    return $metrics;
}
?>

In some cases the (custom) font can be truncated on the side parts, especially the handwritten ones.
To improve the above note from Anonymous on "how to create a header image", I've changed this part of the code:

<?php
/* Create text */
$draw->annotation( $metrics['boundingBox']['y2'], $metrics['ascender'], $text );

/* Create image */
$image->newImage( $metrics['textWidth'] + $metrics['boundingBox']['y2'], $metrics['textHeight'], $background );
?>