__isset() Method

__isset() is a magic method : it is called when the existence of a property has to be checked.

That way, it is possible to create dynamically properties, without hardcoding them.

The __isset() method is usually paired with the __get() and __set() methods.

<?php

class x {
     private $values = ['a' => 1,
                                        'b' => 2,
                                        ];

     function __isset($name) {
             return isset($this->values[$name]);
     }
}

?>

Documentation

Related : Magic Methods, __set() Method, __get() Method