Note that __isset is not called on chained checks. 
If isset( $x->a->b ) is executed where $x is a class with __isset() declared, __isset() is not called.

<?php

class demo
{
    var $id ;
    function __construct( $id = 'who knows' )
    {
        $this->id = $id ;
    }
    function __get( $prop )
    {
        echo "\n", __FILE__, ':', __LINE__, ' ', __METHOD__, '(', $prop, ') instance ', $this->id ;
        return new demo( 'autocreated' ) ; // return a class anyway for the demo
    }
    function __isset( $prop )
    {
        echo "\n", __FILE__, ':', __LINE__, ' ', __METHOD__, '(', $prop, ') instance ', $this->id ;
        return FALSE ;
    }
}
$x = new demo( 'demo' ) ;
echo "\n", 'Calls __isset() on demo as expected when executing isset( $x->a )' ;
$ret = isset( $x->a ) ;
echo "\n", 'Calls __get() on demo without call to __isset()  when executing isset( $x->a->b )' ;
$ret = isset( $x->a->b ) ;
?>

Outputs

Calls __isset() on demo as expected when executing isset( $x->a )
C:\htdocs\test.php:31 demo::__isset(a) instance demo
Calls __get() on demo without call to __isset()  when executing isset( $x->a->b )
C:\htdocs\test.php:26 demo::__get(a) instance demo
C:\htdocs\test.php:31 demo::__isset(b) instance autocreated