Will isset() return false if I assign NULL to a variable?

bool isset ( mixed $var [, mixed $var [, $... ]] )

Determine if a variable is set and is not NULL.

If a variable has been unset with unset(), it will no longer be set. isset() will return FALSE if testing a variable that has been set to NULL. Also note that a NULL byte ("\0") is not equivalent to the PHP NULL constant.

Return values

Returns TRUE if var exists and has value other than NULL, FALSE otherwise.

From the manual. Examples on the same page.

Yes - from the ISSET() documentation:

$foo = NULL;
var_dump(isset($foo));   // FALSE

/* Array example */
$a = array ('test' => 1, 'hello' => NULL);

var_dump(isset($a['test']));            // TRUE
var_dump(isset($a['foo']));             // FALSE
var_dump(isset($a['hello']));           // FALSE

If you need to check if an array contains a key but its value is null, you can use array_key_exists

<?php

$array = [
    'qwe' => null,
    'foo' => 123,
];

var_dump(isset($array['foo']));            // bool(true)
var_dump(array_key_exists('foo', $array)); // bool(true)

var_dump(isset($array['qwe']));            // bool(false)
var_dump(array_key_exists('qwe', $array)); // bool(true) <---

var_dump(isset($array['bar']));            // bool(false)
var_dump(array_key_exists('bar', $array)); // bool(false)