I noticed that PHP returns that a variable was not started if it has no value assigned to it. In my opinion, it does not make sense because the variable has already started and waits for a value.
Example:
class Users {
public $username;
public function __construct() {
if(isset($this->username))
echo 'variável foi iniciada';
else
echo 'variável não foi iniciada';
}
}
What is the reason for this?
If parameters are passed, then isset will return TRUE , only if all parameters are set.
An uninitialized variable is equivalent to the PHP NULL constant. Then the function isset will return FALSE .
If you are dealing with object properties and want to "evaluate" the value of NULL , you can use: property_exists instead of isset :
<?php
class minhaClass{
public $mine;
private $xpto;
static protected $teste;
function teste() {
var_dump(property_exists($this, 'xpto')); //true
}
}
var_dump(property_exists('minhaClass', 'mine')); //true
var_dump(property_exists(new minhaClass, 'mine')); //true
var_dump(property_exists('minhaClass', 'xpto')); //true, para PHP 5.3.0
var_dump(property_exists('minhaClass', 'bar')); //false
var_dump(property_exists('minhaClass', 'test')); //true, para PHP 5.3.0
minhaClass::teste();
?>
To complement, see this table below: F = false T = true
isset is_null ===null ==null empty
null | F | T | T | T | T |
unset | F | T | T | T | T |
"" | T | F | F | T | T |
[] | T | F | F | T | T |
0 | T | F | F | T | T |
false | T | F | F | T | T |
true | T | F | F | F | F |
1 | T | F | F | F | F |
<?php
class minhaClass{
public $mine;
private $xpto;
static protected $teste;
function teste() {
var_dump(property_exists($this, 'xpto')); //true
}
}
var_dump(property_exists('minhaClass', 'mine')); //true
var_dump(property_exists(new minhaClass, 'mine')); //true
var_dump(property_exists('minhaClass', 'xpto')); //true, para PHP 5.3.0
var_dump(property_exists('minhaClass', 'bar')); //false
var_dump(property_exists('minhaClass', 'test')); //true, para PHP 5.3.0
minhaClass::teste();
?>
| T | F | F | F | F |
There is a difference between declaring a variable and assigning a value to it.
Declaring means that you have indicated that a variable will exist in a given context. But only this. In PHP if the variable does not have a value assigned, it is null.
Only when you assign a value does the variable become usable.
In your example, the variable was only declared and not initialized.
I think it's just a problem of naming and understanding the isset method.
From documentation :
Determine if a variable is set and is not NULL.
If the 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 character ("\ 0") is not equivalent to the PHP NULL constant.
You mentioned the word "started". In common understanding of the word, a variable started is a variable that had some value assigned . In the example shown by you, this did not actually happen.
But confusion is normal. In the Java world, a variable can be considered started by assigning the null value to it, unlike the isset method of PHP that does not consider a null variable to be started.
The variable is just a reference to a valor/objeto .
There is a difference between declaring a variable and initializing a variable . In your example you declared the variable but did not initialize it. That is, you did not assign any valor/objeto to it. So its value is still null .
In PHP, variables are evaluated as false by isset in two cases:
null . For the two cases below, the evaluation of isset will return false .
var_dump(isset($a));
$b = null;
var_dump(isset($b));
Additional detail : It is not related to the question, but it is important to note that, in the case of checking indexes of arrays , it is not recommended for some cases to use isset but of array_keys_exists .
Example 1:
$a = array(1 => 'um', 2 => null);
var_dump(isset($a[1])); // bool(true);
var_dump(isset($a[2])); // bool(false);
Example 2:
var_dump(array_keys_exists(1, $a)); // bool(true);
var_dump(array_keys_exists(2, $a)); // bool(true);
As I said, for null values, isset will return false as well.
Note that, array_keys_exists checks for the existence of the index, rather than verifying that the value is null .