The 'at' sign (@) variable prefix

I learned through osmosis that I could use the '@' sign to prevent an error with an uncertain variable. For example, if there is no $_GET[] value in this line, $query=$_GET["q"]; I will get an error, but if I prefix the value with '@', $query=@$_GET["q"]; and no value is available, a null string is returned, and no error is generated. This is great, but I'd like to see it in writing. I can't find this feature in the documentation. Can someone steer me to it? -- Dave