What about the If else problem with PHP?

What if else problem with PHP???
I am a novice, today in the revision of a PHP source code because of the need to use a PHP if else problem, but also hope that you help the heroes.

LINK.PHP?ID=ABC when there is no ABC, the value of the ID is null when the parameter is incorrect, when the value of the ID is not empty, the LINK.PHP?ID=ABC is executed.

How do you write this statement?

------Solution--------------------
if (Isset ($_get[' id ')) {
Have ID

}else{
No ID hint code
}
------Solution--------------------

PHP Code

if (Isset ($_get[' id ')) {    //have ID   }else{//    no ID  hint code}
