$source_file = "images.jpg";

$im = ImageCreateFromJpeg($source_file);

$imgw = imagesx($im);

$imgh = imagesy($im);

$r = array();

$g = array();

$b = array();

$c = 0;

for ($i=0; $i<$imgw; $i++)

for ($j=0; $j<$imgh; $j++)

$rgb = ImageColorAt($im, $i, $j);

$r[$i][$j] = ($rgb >> 16) & 0xFF;

$g[$i][$j] = ($rgb >> 8) & 0xFF;

$b[$i][$j] = $rgb & 0xFF;

if ($r[$i][$j] == $g[$i][$j] && $r[$i][$j] == $b[$i][$j])

$c++;

if ($c == ($imgw*$imgh))

echo "The image is grayscale.";

echo "The image is NOT grayscale.";

After running the program you will get the following output