PHP Fatal error: Cannot access empty property

问题:

I'm new to php and I have executed below code.

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 

回答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

回答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; 

回答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.

回答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:

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

回答5:

Interesting:

1. You declared an array var $my_value = array();
2. Pushed value into it $a->my_value[] = 'b';
3. Assigned a string to variable. (so it is no more array) $a->set_value ('c');
4. 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.

回答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 

回答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!

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