Question

Is undefined a data-type in php?
Also, how does one check for it (on a variable, is or is not undefined)?

Answer 1

There is no “undefined” data type in PHP. You can check for a variable being set with isset, but this cannot distinguish between a variable not being set at all and it having a null value:

var_dump(isset($noSuchVariable)); // false

$nullVariable = null;
var_dump(isset($nullVariable)); // also false

However, there is a trick you can use with compact that allows you to determine if a variable has been defined, even if its value is null:

var_dump(!!compact('noSuchVariable')); // false
var_dump(!!compact('nullVariable')); // true

Live example.

Both isset and the compact trick also work for multiple variables at once (use a comma-separated list).

You can easily distinguish between a null value and total absence when dealing with array keys:

$array = array('nullKey' => null);

var_dump(isset($array['nullKey'])); // false
var_dump(array_key_exists($array, 'nullKey')); // true

Live example.

When dealing with object properties there is also property_exists, which is the equivalent of array_key_exists for objects.