The ImageMagick PHP function for getCompression returns an integer representing the value associated with the ImageMagick Compression constant.  You'll get numbers from 0 to 13, each representing a different particular type of compression.  If printed out, the ImageMagick constants for compression come out as...

imagick::COMPRESSION_UNDEFINED    0
imagick::COMPRESSION_NO    1
imagick::COMPRESSION_BZIP    2
imagick::COMPRESSION_DXT1    3
imagick::COMPRESSION_DXT3    4
imagick::COMPRESSION_DXT5    5
imagick::COMPRESSION_FAX    6
imagick::COMPRESSION_GROUP4    7
imagick::COMPRESSION_JPEG    8
imagick::COMPRESSION_JPEG2000    9
imagick::COMPRESSION_LOSSLESSJPEG    10
imagick::COMPRESSION_LZW    11
imagick::COMPRESSION_RLE    12
imagick::COMPRESSION_ZIP    13

Every time I have used this, whether on a jpeg image, a png image, a gif image, or a bmp image, it has always returned '0' as a value.  There's a good chance that this is simply a value that is set by means of get/set, as opposed to actually producing values for a given image.

Some sample code :

<?php

            // Author: [email protected]
    
        // Imagick Type
        // ---------------------------------------------

    $imagick_type = new Imagick();
    
        // Open File
        // ---------------------------------------------
        
    $file_to_grab = "image_workshop_directory/test.bmp";
    
    $file_handle_for_viewing_image_file = fopen($file_to_grab, 'a+');
    
        // Grab File
        // ---------------------------------------------

    $imagick_type->readImageFile($file_handle_for_viewing_image_file);
    
        // Get Quantum Range
        // ---------------------------------------------
        
    $imagick_type_compression = $imagick_type->getCompression();
    
        // Print Results
        // ---------------------------------------------

    print("<pre>");
    print($imagick_type_compression);
    print("</pre>");

?>