Null and missing values in PHP

Null has one value

The null type has exactly one value: null. The constant is case-insensitive, so NULL denotes the same value. An undefined or unset variable resolves to null, but reading an undefined variable directly also issues a warning.

PHP

<?php
$value = null;
var_dump($value, $value === false, $value === 0);

Output

NULL
bool(false)
bool(false)

null is distinct from false, 0, and the empty string.
Assigning null and removing a variable with unset are different operations.

Check whether a value is set

isset returns false when a variable is undefined or has the value null. It returns true for an existing variable holding an empty string. This excerpt retains the manual's null check.

PHP

<?php
$foo = NULL;
var_dump(isset($foo));   // FALSE

Output

bool(false)

Unsetting removes the variable

unset removes a variable; it is a language construct rather than a cast. Use is_null to check whether an existing value is null. isset alone cannot distinguish an undefined variable from a variable explicitly assigned null.

PHP

<?php
$value = null;
var_dump(is_null($value), isset($value));
unset($value);
var_dump(isset($value));

Output

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

The old (unset) cast is removed in PHP 8.0 and is not available on this course's PHP 8.4 baseline.

Check your understanding

What does var_dump report?

<?php
$value = null;
var_dump(isset($value));

bool(true) bool(false)

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