After operations that change the crop of the image, like trimImage does, IM preserves the old canvas and positioning info. If you need to do additional operations on the image based on the new size, you'll need to reset this info with setImagePage. This is the equivalent of the +repage command line argument.

<?php
$im->trimImage(0);
$im->setImagePage(0, 0, 0, 0);
?>

The command for trim can return info which tells you the coordinates of where the image was cropped and the new dimensions of the trimmed image. I couldn't find an example for how to do that with this class, so here's how I did it:

<?php

// Create and trim image
$image = new Imagick('image.png');
$image->trimImage(0.3);

/**
* Imagick:: getImagePage() gives us an array with the width, height, x and y coordinates of the trimmed image
* However, the width and height are of the original image
*/
$imagePage = $image->getImagePage();
list($x, $y) = array($imagePage['x'], $imagePage['y']);

// So we need to reset the image page to get the right height and width of the trimmed image
$image->setImagePage(0, 0, 0, 0);
list($width, $height) = array($image->width, $image->height);

?>

This will leave you with two variables for the width and height of the trimmed image, plus two variables for the x-coordinate and y-coordinate of the trimmed area relative to the original image.

This information is useful when the image you're trimming is the difference between two images (e.g. from Imagick::compareImages(), and you want to crop the original image to the same size and position as the difference.

fuzz needs getQuantum
/**
 * @param string $imagePath
 * @param float  $fuzz 0.1=10%
 * @return bool
 * @throws ImagickException
 */
public static function trimImage($imagePath, $fuzz = 0.1)
{
    if (empty($imagePath)) {
        return false;
    }
    $imagePath = realpath($imagePath);
    if (!is_file($imagePath)) {
        return false;
    }
    if (!class_exists('Imagick')) {
        return false;
    }
    $im = new Imagick($imagePath);
    $result = $im->trimImage($fuzz * $im::getQuantum());
    if (!$result) {
        return false;
    }

    return $im->writeImage($imagePath);
}

This method requires ImageMagick version >= 6.2.8