PHP function imagettftext() to write text with smi

问题:

I am using imagettftext function to write text on image but my text contain smileys and it replace smileys to square box

Here is mycode:

$black = imagecolorallocate ( $main_img, 0x00, 0x00, 0x00 );
$font_path = "Arial.ttf";
imagettftext ( $main_img, 14, 0, 73, 685, $black, $font_path, $text );

Text Sample:

test testt