Problems with adding text to image

I have a peice of code that adds some text to images on-the-fly for my image galleries. It works perfectly in normal instances. However when I pass a filename to my function that contains a  ' (single quote) character the function does not return the new image with the text on - it doesn't return anything and I get an image not found box in my browser. It doesn't matter where in the filename the ' is (whether it be in a directory path or the filename itself).

I've tried escaping the ' and I've tried doubling the ' to '' but neither seem to work.

I'm running Suse Linux 8.1, Apache 1.3.27, php 4.3.0 compiled with in-built GD library and Mysql 3.23.55.

Here are the relevant code snippets:

It is called like this from my showphoto.php script

print "<img src=\"/includes/addtext.php?img=../photos/".urlencode(addslashes($myrow["url"]))."&txt=".urlencode("Copyright ".$myrow["taken_by"])."&loc=".$loc."&col=".$col."\" alt=\"".$myrow["alt_text"]."\">";

where
img = image name (including path)
txt = text to show
loc = location of text
col = colour of text
$myrow is an array with results from a db query in it.

and here is addtext.php
<?php
     $image = "./".$_GET["img"];
     
     $txt = $_GET["txt"];
     $pos = $_GET["loc"];
     $col = $_GET["col"];
     
     // Strip out any html tags from the txt text string (remove any hrefs or email links)
     $txt = preg_replace("/<\/?\w+[^>]*>/","",$txt);

     $fnt = 2;
     $pos = strtolower($pos);
     $col = strtolower($col);
     $src_img = ImageCreateFromJpeg($image);
     $img_x = ImageSX($src_img);
     $img_y = ImageSY($src_img);

     $white = ImageColorAllocate($src_img, 255, 255, 255);
     $black = ImageColorAllocate($src_img, 0, 0, 0);

     $font_w = imagefontwidth($fnt);
     $font_h = imagefontheight($fnt);
     $slen = strlen($txt);
     $ad_text = "our web site";
     $ad_len = strlen($ad_text);

     switch ($col) {
          case 'white':
          case 'wh':
          case 'w':
               $txt_col = $white;
               break;
          case 'black':
          case 'bl':
          case 'b':
               $txt_col = $black;
               break;
          default:
               $txt_col = $black;
          }

     switch ($pos) {
         case 'bl':
             $str_x_pos = 5;
             $ad_str_x_pos = 5;
             $str_y_pos = ($img_y - (20 + $font_h));
          break;
         case 'br':
             $str_x_pos = ($img_x - (($slen * $font_w) +5));
             $ad_str_x_pos = ($img_x - (($ad_len * $font_w) +5));
             $str_y_pos = ($img_y - (20 + $font_h));
          break;
         case 'tl':
             $str_x_pos = 5;
             $ad_str_x_pos = 5;
             $str_y_pos = 5;
          break;
         case 'tr':
             $str_x_pos = ($img_x - (($slen * $font_w) +5));
             $ad_str_x_pos = ($img_x - (($ad_len * $font_w) +5));
             $str_y_pos = 5;
          break;
         default:
             $str_x_pos = 5;
             $str_y_pos = ($img_y - (5 + $font_h));
          }

     imagestring($src_img, $fnt, $str_x_pos, $str_y_pos, $txt, $txt_col);
     imagestring($src_img, $fnt, $ad_str_x_pos, $str_y_pos+15, $ad_text, $txt_col);

     ImageJpeg($src_img,'',100);
     ImageDestroy($src_img);

     return(1);

?>

So, does anyone have any ideas how to make this code work with filenames that have single quotes in them? (Remember it works perfectly with filenames that don't have ' in them)

Thanks
Andrew