Description:
------------
ImageMagick introduced the LinearGray colorspace (sGray) in 6.9.9-28+ and 7.0.7-16+ (http://www.imagemagick.org/discourse-server/viewtopic.php?f=4&t=33265)


sGray CONST didnt get implemented till now in imagick.


Test script:
---------------
identify -verbose image.png | grep -E 'Version|Colorspace'
  Colorspace: sGray
  Version: ImageMagick 6.9.9-28 Q16 amd64 2017-12-27 http://www.imagemagick.org


<?php
$im = new Imagick('image.png');
$color = $im->getImageColorspace();

//Return of getImageColorspace(Integer)
echo $color;


//Print all available Colorspaces
$reflection = new \ReflectionClass("\Imagick");
$colorspaces = array_flip(
    array_filter(
        $reflection->getConstants(),
        function ($k) {
            return mb_strpos($k, "COLORSPACE") !== false;
        },
        ARRAY_FILTER_USE_KEY
    )
); 
print_r($colorspaces);
?>

Expected result:
----------------
Prints out:
35

Array ( [0] => COLORSPACE_UNDEFINED [1] => COLORSPACE_RGB [2] => COLORSPACE_GRAY [3] => COLORSPACE_TRANSPARENT [4] => COLORSPACE_OHTA [5] => COLORSPACE_LAB [6] => COLORSPACE_XYZ [7] => COLORSPACE_YCBCR [8] => COLORSPACE_YCC [9] => COLORSPACE_YIQ [10] => COLORSPACE_YPBPR [11] => COLORSPACE_YUV [12] => COLORSPACE_CMYK [13] => COLORSPACE_SRGB [14] => COLORSPACE_HSB [15] => COLORSPACE_HSL [16] => COLORSPACE_HWB [17] => COLORSPACE_REC601LUMA [19] => COLORSPACE_REC709LUMA [21] => COLORSPACE_LOG [22] => COLORSPACE_CMY [23] => COLORSPACE_LUV [24] => COLORSPACE_HCL [25] => COLORSPACE_LCH [26] => COLORSPACE_LMS [27] => COLORSPACE_LCHAB [28] => COLORSPACE_LCHUV [29] => COLORSPACE_SCRGB [30] => COLORSPACE_HSI [31] => COLORSPACE_HSV [32] => COLORSPACE_HCLP [33] => COLORSPACE_YDBDR [18] => COLORSPACE_REC601YCBCR [20] => COLORSPACE_REC709YCBCR [34] => COLORSPACE_XYY [35] => COLORSPACE_SGRAY )

Actual result:
--------------
Prints out:
35

Array ( [0] => COLORSPACE_UNDEFINED [1] => COLORSPACE_RGB [2] => COLORSPACE_GRAY [3] => COLORSPACE_TRANSPARENT [4] => COLORSPACE_OHTA [5] => COLORSPACE_LAB [6] => COLORSPACE_XYZ [7] => COLORSPACE_YCBCR [8] => COLORSPACE_YCC [9] => COLORSPACE_YIQ [10] => COLORSPACE_YPBPR [11] => COLORSPACE_YUV [12] => COLORSPACE_CMYK [13] => COLORSPACE_SRGB [14] => COLORSPACE_HSB [15] => COLORSPACE_HSL [16] => COLORSPACE_HWB [17] => COLORSPACE_REC601LUMA [19] => COLORSPACE_REC709LUMA [21] => COLORSPACE_LOG [22] => COLORSPACE_CMY [23] => COLORSPACE_LUV [24] => COLORSPACE_HCL [25] => COLORSPACE_LCH [26] => COLORSPACE_LMS [27] => COLORSPACE_LCHAB [28] => COLORSPACE_LCHUV [29] => COLORSPACE_SCRGB [30] => COLORSPACE_HSI [31] => COLORSPACE_HSV [32] => COLORSPACE_HCLP [33] => COLORSPACE_YDBDR [18] => COLORSPACE_REC601YCBCR [20] => COLORSPACE_REC709YCBCR [34] => COLORSPACE_XYY )