Imagine the following small scriptlet:

$fd = fopen( $file, "r" );
$contents = fread( $fd, filesize( $file) );
fclose( $fd );
if (strpos($contents, "GIF87a")==0)
  {$kind="GIF87a";}
else
  {if (strpos($contents, "GIF89a")==0) 
     {$kind="GIF89a";}
   else 
     {if (strpos($contents, "JFIF")==6) 
        {$kind="JPEG";}
      else 
        {$kind="no_image";}}}
infobox("The uploaded file ($file_name) is $kind");

---

Problem is: strpos() returns 0 when either $contents starts with "GIF89a" or when "GIF89a" is not included in $contents at all.
Suggestion: start counting positions with one, or use other value for "false"?