The getQuantumDepth works much like the other "Depth" functions and much like the getQuantumRange function.  Like the Depth functions, it returns a value indicating the number of bits to store the unique color values -- 16-bit = 2^16 unique colors, which is 65,535 (counting starts at '0' in binary, as opposed to '1' in decimal).  Like the getQuantumRange function, it returns an array with two values, one being a long integer ('quantumDepthLong') and the other being a string ('quantumDepthLong').  The only difference, besides the variable types, is that the string version has a 'Q' prefixed to it.

Here's some sample code and the results, given a color, BMP file photograph that is 600x450 pixels...

<?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 Depth
        // ---------------------------------------------
        
    $imagick_type_quantum_depth = $imagick_type->getQuantumDepth();
    
        // Print Results
        // ---------------------------------------------

    print("<pre>");
    print_r($imagick_type_quantum_depth);
    print("</pre>");

?>

Output:

Array
(
    [quantumDepthLong] => 16
    [quantumDepthString] => Q16
)