PHP Fatal error: Cannot access empty property - php

Question

I'm new to php and I have executed below code.
<?php
class my_class{
var $my_value = array();
function my_class ($value){
$this->my_value[] = $value;
}
function set_value ($value){
// Error occurred from here as Undefined variable: my_value
$this->$my_value = $value;
}
}
$a = new my_class ('a');
$a->my_value[] = 'b';
$a->set_value ('c');
$a->my_class('d');
foreach ($a->my_value as &$value) {
echo $value;
}
?>
I got below errors. What could be the error?
Notice: Undefined variable: my_value in C:\xampp\htdocs\MyTestPages\f.php on line 15
Fatal error: Cannot access empty property in C:\xampp\htdocs\MyTestPages\f.php on line 15

Answer 1

You access the property in the wrong way. With the $this->$my_value = .. syntax, you set the property with the name of the value in $my_value. What you want is $this->my_value = ..
$var = "my_value";
$this->$var = "test";
is the same as
$this->my_value = "test";
To fix a few things from your example, the code below is a better aproach
class my_class {
public $my_value = array();
function __construct ($value) {
$this->my_value[] = $value;
}
function set_value ($value) {
if (!is_array($value)) {
throw new Exception("Illegal argument");
}
$this->my_value = $value;
}
function add_value($value) {
$this->my_value = $value;
}
}
$a = new my_class ('a');
$a->my_value[] = 'b';
$a->add_value('c');
$a->set_value(array('d'));
This ensures, that my_value won't change it's type to string or something else when you call set_value. But you can still set the value of my_value direct, because it's public. The final step is, to make my_value private and only access my_value over getter/setter methods

Answer 2

First, don't declare variables using var, but
public $my_value;
Then you can access it using
$this->my_value;
and not
$this->$my_value;

Answer 3

To access a variable in a class, you must use $this->myVar instead of $this->$myvar.
And, you should use access identifier to declare a variable instead of var.
Please read the doc here.

Answer 4

As I see in your code, it seems you are following an old documentation/tutorial about OOP in PHP based on PHP4 (OOP wasn't supported but adapted somehow to be used in a simple ways), since PHP5 an official support was added and the notation has been changed from what it was.
Please see this code review here:
<?php
class my_class{
public $my_value = array();
function __construct( $value ) { // the constructor name is __construct instead of the class name
$this->my_value[] = $value;
}
function set_value ($value){
// Error occurred from here as Undefined variable: my_value
$this->my_value = $value; // remove the $ sign
}
}
$a = new my_class ('a');
$a->my_value[] = 'b';
$a->set_value ('c'); // your array variable here will be replaced by a simple string
// $a->my_class('d'); // you can call this if you mean calling the contructor
// at this stage you can't loop on the variable since it have been replaced by a simple string ('c')
foreach ($a->my_value as &$value) { // look for foreach samples to know how to use it well
echo $value;
}
?>
I hope it helps

Answer 5

Interesting:
You declared an array var $my_value = array();
Pushed value into it $a->my_value[] = 'b';
Assigned a string to variable. (so it is no more array) $a->set_value ('c');
Tried to push a value into array, that does not exist anymore. (it's string) $a->my_class('d');
And your foreach wont work anymore.

Answer 6

This way you can create a new object with a custom property name.
$my_property = 'foo';
$value = 'bar';
$a = (object) array($my_property => $value);
Now you can reach it like:
echo $a->foo; //returns bar

Answer 7

I realise this answer is not a direct response to the problem described by the OP, but I found this question as a result of searching for the same error message. I thought it worth posting my experience here just in case anybody is muddling over the same thing...
You can encounter the error in question as a result of a poorly formatted for loop over an associative array. In a fit of bone-headedness, I was using -> instead of => in my for statement:
foreach ($object->someArray as $key->$val) {
// do something
}
Of course, I should have had:
foreach ($object->someArray as $key=>$val) {
// do something
}
I confused myself at first, thinking the reported error was referring to the someArray property!