Using php imagettftext with Wingdings font results in only boxes being displayed

Question

In my php application I am using imagettftext() successfully with several font including OCR-A and others, but when I use wingding.ttf as the font I only get squares (character not found) as the result eg:imagettftext($image, 8, 0, 0, 10, 1, $font, 'ê'); to display the bold arrow down image.

Is Wingdings only for Windows OS?

Thanks.