Change size of text with built in font

[robertaccettura]

I'm learning a little GD (wish I did earlier, this is real useful stuff.

So I decided to do a little demo:

		$watermarkText="Watermark";
		$font = 1; 
		$textWidth = ImageFontWidth($font) * strlen($watermarkText); 
		$textHeight = ImageFontHeight($font); 
		$left = (($width-$textWidth) / 2); 
		$top = (($height-$textHeight) / 2); 
	 	$textColor = imagecolorallocate($Main_Image,102,204,255); 

That does a fine job. But how can I adjust the size of the font? I would like something a bit bigger. Say size 18 font.

I also want to keep all my variables (textWidth, textHeight etc.)

Cool tool.