PHP: Add text to transparent PNG image

PHP: Add text to transparent PNG image

<?php 
$originalImage = "group_icon.png";
if(file_exists($originalImage)) {
	$im = imagecreatefrompng($originalImage);
	imagesavealpha($im, true); // important to keep the png's transparency 
	if(!$im) {
		die("im is null");
	}
	$black = imagecolorallocate($im, 0, 0, 0);
	$width = 36; // the width of the image
	$height = 36; // the height of the image
	$font = 2; // font size
	$digit = $i; // digit
	$leftTextPos = 19 - (strlen($digit)*3);
	$outputImage = "group_icon_".$digit.".png";
	imagestring($im, $font, $leftTextPos, 9, $digit, $black);
	imagepng($im, $outputImage, 0);
	imagedestroy($im);
}
?>

$i; not defined.