What Is System Identifier For General Entity

In my PHP Script I have used a name/value pair to pass the value of certain variables into a new window.

For example: I have used code like :

<a href= "show_image.php?image_url=$current_image_url">Click here to see full size image!</a>

The value of $current_image_url is substituted by the actual url of the image file.

Now when I validate that page I'm getting the following error message :

cannot generate system identifier for general entity "fs_image_url"

Can anybody suggest how to resolve that error so that the webpage passes w3c validation test?

Regards,

Sid