php undefined

1if (isset($variable)) { /* do something */ };

1/**
2 * Returns a bool (true or false)
3 */
4isset($x);
5/**
6 * Examples
7 */
8$x = 'myValue';
9if(isset($x)){
10	echo 'x is set';
11}
12/**
13 * this will echo out 'x is set'
14 */
15
16
17$x = null;
18if(isset($x)){
19	echo 'x is set';
20}
21/**
22 * This will NOT echo out 'x is set'
23 */
24
25
26if(isset($y)){
27 	echo 'y is set'; 
28}
29/**
30 * This will NOT echo out 'y is set'
31 */
32
33

1$myId = 0;
2function run(){
3  global $myId;
4	echo $myId;
5}