Adjusts the levels of an image by scaling the colors falling between specified white and black points to the full available quantum range. The parameters provided represent the black, mid, and white points. The black point specifies the darkest color in the image. Colors darker than the black point are set to zero. Mid point specifies a gamma correction to apply to the image. White point specifies the lightest color in the image. Colors brighter than the white point are set to the maximum quantum value.

The 'Reversed level adjustment' function isn't exposed as function in the ImageMagick library so isn't usable in Imagick.

Instead you can use the evaluateImage function to achive the same effect. Please see the levelizeImage tutorial.
Black point
WhitePoint
Gamma
Adjusts the levels of an image by scaling the colors falling between specified white and black points to the full available quantum range. The parameters provided represent the black, mid, and white points. The black point specifies the darkest color in the image. Colors darker than the black point are set to zero. Mid point specifies a gamma correction to apply to the image. White point specifies the lightest color in the image. Colors brighter than the white point are set to the maximum quantum value.

The 'Reversed level adjustment' function isn't exposed as function in the ImageMagick library so isn't usable in Imagick.

Instead you can use the evaluateImage function to achive the same effect. Please see the levelizeImage tutorial.
Parameters
blackPointfloatThe image black point
gammafloatThe gamma value
whitePointfloatThe image white point
channelintProvide any channel constant that is valid for your channel mode. To apply to more than one channel, combine channeltype constants using bitwise operators. Refer to this list of channel constants.
function levelImage($blackPoint, $gamma, $whitePoint)
{
    $imagick = new \Imagick();
    $imagick->newPseudoimage(500, 500, 'gradient:black-white');

    $imagick->setFormat('png');
    $quantum = $imagick->getQuantum();
    $imagick->levelImage($blackPoint / 100, $gamma, $quantum * $whitePoint / 100);

    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}