NULL & isset / empty

null is PHP's way of representing "no value." It is its own type with exactly one possible value, written as the bare keyword null (case-insensitive, though lowercase is the convention). A variable can be explicitly set to null, or it can simply never have been assigned at all — and PHP gives you three different functions, isset(), empty(), and is_null(), to distinguish between those situations, each with subtly different rules that are worth learning precisely rather than guessing at.

What null actually means

Assigning and checking null

PHP

<?php
$middleName = null;

var_dump($middleName);
var_dump($middleName === null);
var_dump(gettype($middleName));

NULL
bool(true)
string(4) "NULL"

A variable holding null still exists — it has been assigned a value, that value just happens to be "nothing." This is different from a variable that was never assigned to at all, which is the first source of confusion this page untangles.

is_null()

is_null($value) is the most literal check: it returns true only when the value is exactly null, and behaves identically to $value === null. It does not care whether the variable was ever defined — checking an undefined variable with is_null() still works, but as of PHP 8 it raises a warning for accessing an undefined variable, just like reading it directly would.

is_null() vs strict comparison

PHP

<?php
$a = null;
$b = 0;

var_dump(is_null($a));      // bool(true)
var_dump(is_null($b));      // bool(false) - 0 is not null
var_dump($a === null);      // bool(true) - identical check

bool(true)
bool(false)
bool(true)

isset()

isset($variable) answers a slightly different question: "does this variable exist, and is its value anything other than null?" A variable that was never assigned returns false. A variable explicitly set to null also returns false — which surprises people expecting isset() to just mean "has this been assigned." Critically, isset() never raises a warning or notice even when the variable does not exist, which is exactly why it is the standard guard for optional data.

isset() across three situations

PHP

<?php
$city = "Nairobi";
$region = null;
// $country was never assigned

var_dump(isset($city));    // bool(true)
var_dump(isset($region));  // bool(false) - explicitly null
var_dump(isset($country)); // bool(false) - undefined, no warning

bool(true)
bool(false)
bool(false)

isset() also works on array keys and object properties, and this is where it is used most often in real code: isset($_GET['page']) safely checks for a query-string parameter that may or may not be present, without triggering a warning for the missing key.

empty()

empty($variable) asks a broader question again: "is this variable either unset, or does it hold a falsy value?" It is essentially !isset($variable) || !$variable, bundled into one function that also never warns about undefined variables.

empty() catches more cases than isset()

PHP

<?php
$votes = 0;
$name = "";
$items = ["apple"];

var_dump(empty($votes));   // bool(true) - 0 is falsy
var_dump(empty($name));    // bool(true) - "" is falsy
var_dump(empty($items));   // bool(false) - non-empty array is truthy
var_dump(empty($missing)); // bool(true) - undefined, no warning

bool(true)
bool(true)
bool(false)
bool(true)

Truth table: isset() vs empty() vs is_null()

Value	isset()	empty()	is_null()
undefined variable	false	true	true (with warning)
`null`	false	true	true
`0`	true	true	false
`""`	true	true	false
`"0"`	true	true	false
`false`	true	true	false
`[]`	true	true	false
`"hello"`	true	false	false

Reading an undefined variable directly triggers a warning in PHP 8

Prior to PHP 8, accessing an undefined variable just silently returned `null` with a low-priority notice. As of PHP 8.0, this was promoted to a full `E_WARNING`, which is visible by default in most setups. Always check with `isset()` (or use the null coalescing operator) before reading a variable, array key, or property that might not exist, instead of reading it and hoping it is set.

A preview of the null coalescing operator

The pattern isset($value) ? $value : $default is common enough that PHP 7 introduced a dedicated shorthand: the null coalescing operator, ??. It evaluates its left-hand side and returns it if it exists and is not null, otherwise it returns the right-hand side — with no warning either way.

?? replacing an isset() ternary

PHP

<?php
$page = $_GET['page'] ?? 1;
echo $page;