isset vs empty vs is_null

I'm trying to write a script that when a user uploads a file and does not enter a name an error is returned. I've tried using is_null, empty, and isset and they all do not work. Eg, below, is_null returns an error even when a name is entered. Can anyone help?

$caption = $_REQUEST[$name_input_name];

if(is_null($caption)) {
    $file->error = 'Please Enter a Title';
    return false;
}

Solution 1:

is_null() emits a WARNING if variable is not set, but isset() and empty() don't.

$a - variable with not null value (e.g. TRUE)
$b - variable with null value. `$b = null;`
$c - not declared variable
$d - variable with value that cast to FALSE (e.g. empty string, FALSE or empty array)
$e - variable declared, but without any value assigned
$a->a - declared, but not assigned object property. (`public $a;`)
A::$a - declared, but not assigned static class property.

         |   $a  |   $b  |   $c  |   $d  |   $e  | $a->a | A::$a |
---------+-------+-------+-------+-------+-------+-------+-------+
is_null()| FALSE | TRUE  |TRUE*W | FALSE | TRUE*W| TRUE  | TRUE  |
---------+-------+-------+-------+-------+-------+-------+-------+
isset()  | TRUE  | FALSE | FALSE | TRUE  | FALSE | FALSE | FALSE |
---------+-------+-------+-------+-------+-------+-------+-------+
empty()  | FALSE | TRUE  | TRUE  | TRUE  | TRUE  | TRUE  | TRUE  |
---------+-------+-------+-------+-------+-------+-------+-------+
null === | FALSE | TRUE  |TRUE*W | FALSE | TRUE*W| TRUE  | TRUE  |
---------+-------+-------+-------+-------+-------+-------+-------+
null ==  | FALSE | TRUE  |TRUE*W | TRUE  | TRUE*W| TRUE  | TRUE  |
---------+-------+-------+-------+-------+-------+-------+-------+

TRUE*W - function return TRUE, but same time emits WARNING.

On empty() function documentation page you can read, that:

The following things are considered to be empty:

....

$var; (a variable declared, but without a value)

It can be misleading that code $var; is defining a variable, but does not assign any value to it, but it is wrong. Variable $var is still undefined and type recognize functions, like is_null() emits warnings if you pass $var as an argument.

But it is not right for unsettled class or object properties. Declaring them without assigning some value automatically assigns NULL.

UPD Typed properties in PHP 7.4 DO NOT assigned by NULL by default. If you does not set any value to them, they are considered as unassigned.

Some low level descriptions:

isset() and empty() are core functions, that will be compiled directly to specific opcode according to zval type:

ZEND_ISSET_ISEMPTY_THIS
ZEND_ISSET_ISEMPTY_CV
ZEND_ISSET_ISEMPTY_VAR
ZEND_ISSET_ISEMPTY_DIM_OBJ
ZEND_ISSET_ISEMPTY_PROP_OBJ
ZEND_ISSET_ISEMPTY_STATIC_PROP

Furthermore they will compile by the same function zend_compile_isset_or_empty

Function is_null() is type recognizer function, like is_numeric, is_recource, is_bool, etc. And will be called like user-defined function with opcodes INIT_FCALL_BY_NAME/DO_FCALL_BY_NAME and so.

/* {{{ proto bool is_null(mixed var)
   Returns true if variable is null
   Warning: This function is special-cased by zend_compile.c and so is usually bypassed */
PHP_FUNCTION(is_null)
{
    php_is_type(INTERNAL_FUNCTION_PARAM_PASSTHRU, IS_NULL);
}

Solution 2:

isset() will check if the variable is set, ie

<?php

echo isset($var); // false

$var = 'hello';

empty() will check if the variable is empty, ie

<?php

$emptyString = '';

echo empty($emptyString); // true

is_null() will check for NULL which is different from empty, because it's set to NULL not an empty string. (NULL might be a confusing concept)

Since your title is a string, I think you want to be using empty()

if (!isset($_REQUEST[$name_input_name]) || empty($_REQUEST[$name_input_name])) {
    $file->error = 'Please Enter a Title';
    return false;
}

Solution 3:

PHP empty() vs is_null() vs isset():

If you want to check if there's any value other than null or undefined, use isset($var)
(because !is_null() generates a warning on undefined variables.)

If you want to check if the value is non-blank text or any number including zero, it gets trickier:

if (!empty($v) || (isset($v) && ($v === 0 || $v === '0'))) {
    // $v is non-blank text, true, 0 or '0'
    // $v is NOT an empty string, null, false or undefined
}