The ampersand is causing the break in the URL - PHP populates the $_GET array from the URL. Everything to the right of the question mark goes to the $_GET array, with each pair of key/values.
$_GET["field1"] = "value1"
$_GET["field2"] = "value2"
Now because you have an ampersand in the URL, PHP thinks it is a new variable, which is the reason why the second half is disappearning.
The solution: encode the URL correctly - look at htmlentities(), htmlspecialchars() or urlencode()