(PECL imagick 2.0.0)
Imagick::setImageBias — Sets the image bias for any method that convolves an image
$bias
   )Sets the image bias for any method that convolves an image (e.g. Imagick::ConvolveImage()).
bias
   Returns TRUE on success.
  
Throws ImagickException on error.
Example #1 Imagick::setImageBias()
<?php
//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();
}
?>