Example

//requires ImageMagick version 6.9.0-1 to have an effect on convolveImage
function setImageBias($bias)
{
    $imagick = new \Imagick(realpath("images/stack.jpg"));

    $xKernel = array(
        -0.70, 0, 0.70,
        -0.70, 0, 0.70,
        -0.70, 0, 0.70
    );

    $imagick->setImageBias($bias * \Imagick::getQuantum());
    $imagick->convolveImage($xKernel, \Imagick::CHANNEL_ALL);

    $imagick->setImageFormat('png');
    
    header('Content-type: image/png');
    echo $imagick->getImageBlob();
}