undefined in php code example

Example 1: php check if parameter exists

if(isset($_GET['id']))
{
   $slide = $_GET['id'] // Getting parameter value inside PHP variable
}

Example 2: php see if undefined

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

Example 3: isset in php

/**
 * Returns a bool (true or false)
 */
isset($x);
/**
 * Examples
 */
$x = 'myValue';
if(isset($x)){
	echo 'x is set';
}
/**
 * this will echo out 'x is set'
 */


$x = null;
if(isset($x)){
	echo 'x is set';
}
/**
 * This will NOT echo out 'x is set'
 */


if(isset($y)){
 	echo 'y is set'; 
}
/**
 * This will NOT echo out 'y is set'
 */