[PHP] Issue With GD library

http://scr.hu/1yl7/9608b[/code:5pnrwpy8]

Today I again run code to make some documentation to improve it in future, but there is something weird...
Output image looks like this:
[code:5pnrwpy8]http://scr.hu/1yl7/63kgo[/code:5pnrwpy8]

and here is full code:
[code:5pnrwpy8]
	<?php

			function MakeRandomTxt($lenght = 5){
				$characters = '!@#$%^&*()_+0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
				$charactersLength = strlen($characters);
				$randomString = '';
					for ($i = 0; $i < $lenght; $i++) {
						$randomString .= $characters[rand(0, $charactersLength - 1)];
					}
				return $randomString;
			}

			
		$rnd = MakeRandomTxt();	
			
		$string = $rnd;
		$font_size = rand(24, 48); 
		$width  = 140+imagefontwidth($font_size)*strlen($string); 
		$height = 36+imagefontheight($font_size); 
		$img 	= imagecreate($width,$height); 
		$bg   	= imagecolorallocate($img, rand(0,255), rand(0,255), rand(0,255)); 
		$color 	= imagecolorallocate($img, rand(0,255), rand(0,255), rand(0,255)); 
		$len 	= strlen($string); 
		$ypos 	= 14; 
		
		for($i=0;$i<$len;$i++){ 
		
			$xpos = 64+$i * imagefontwidth($font_size); 
			imagechar($img, $font_size+20, $xpos, rand(2, $ypos), $string, $color); 
			$string = substr($string, 1);    
			
		} 
		header("Content-Type: image/png");
		imagepng($img, './cp/'.$rnd.'.png'); 
		imagedestroy($img); 
		
	?>
[/code:5pnrwpy8]

Pastebin version:
[code:5pnrwpy8]http://pastebin.com/wDvNYwDm[/code:5pnrwpy8]

What causing this problem?