Cannot access child static property from parent static method in PHP

Question

I am having some problem in my PHP code. I have three classes named as

1. DatabaseObject
2. File
3. user_picture

And their inheritance tree from parent to child class is DatabseObect->File->user_picture. Here in user_picture I have a static property $table_name that I am using to know database table name for each class and have some common database functions in DatabaseObject class. Code for a function I am using is here

Code For DatabaseObject Class

public static function find_by_field($field="", $value="")
{
    $result_array = self::find_by_sql("SELECT * FROM ".static::$table_name." WHERE {$field} = '".$value."'"); //Line 52
    if(empty($result_array))
    {
        return false;
    }
    else
    {
        $object = static::instantiate(array_shift($result_array));
        return $object;
    }
}

Code For File Class

public $errors=array();

public $file_before_upload; // General file tyoe that has been uploaded from the form
public $id = 0;
public $size;
public $file_name;
public $file_type;
public $user_id;

protected $temp_path;
//protected static $upload_dir="";

protected $upload_errors = array(
    // http://www.php.net/manual/en/features.file-upload.errors.php
    UPLOAD_ERR_OK               => "No errors.",
    UPLOAD_ERR_INI_SIZE     => "Larger than upload_max_filesize.",
    UPLOAD_ERR_FORM_SIZE    => "Larger than form MAX_FILE_SIZE.",
    UPLOAD_ERR_PARTIAL      => "Partial upload.",
    UPLOAD_ERR_NO_FILE      => "No file.",
    UPLOAD_ERR_NO_TMP_DIR => "No temporary directory.",
    UPLOAD_ERR_CANT_WRITE => "Can't write to disk.",
    UPLOAD_ERR_EXTENSION    => "File upload stopped by extension."
);
public static function get_file_by_user($id = 0){
       return DatabaseObject::find_by_field("user_id",$id); //Line 164
    }

Code for user_picture Class

protected static $table_name = "user_images";
    protected static $db_fields = array('id','file_name','file_type','size','user_id');

    public static $upload_dir ="uploads".DS."user_img";
    protected static $file_upload_limit = 15;


    public static function get_image_by_user($id = 0)
    {
        return File::get_file_by_user($id);  //Line 45
    }

And From where I am calling upper function in user_picture is

$user_image = user_picture::get_image_by_user($session->user_id);

$user_picture_path="";

if(!$user_image)
{
    $user_picture_path = $user_image::upload_dir.DS."default.png";
}
else{
    $user_picture_path = $user_image::file_path(); //Line 28
}

this code gives me the following error

Fatal error: Uncaught Error: Access to undeclared static property: DatabaseObject::$table_name in C:\xampp\htdocs\inventory-management\includes\database\DatabaseObject.php:52 Stack trace:

0 C:\xampp\htdocs\inventory-management\includes\general\File.php(164): DatabaseObject::find_by_field('user_id', '29')

1 C:\xampp\htdocs\inventory-management\includes\user\user_picture.php(45): File::get_file_by_user('29')

2 C:\xampp\htdocs\inventory-management\public\admin\layouts\admin\header\Navigation.php(28): user_picture::get_image_by_user('29')

3 C:\xampp\htdocs\inventory-management\public\admin\layouts\admin\header\admin_header.php(21): require_once('C:\xampp\htdocs...')

4 C:\xampp\htdocs\inventory-management\public\admin\add_page_category.php(44): require_once('C:\xampp\htdocs...')

5 {main} thrown in C:\xampp\htdocs\inventory-management\includes\database\DatabaseObject.php on line 52

Please if anybody can help!

Thanks,

Answer 1

You have to call self instead of Database:: and File::

public static function get_image_by_user($id = 0)
{
    return self::get_file_by_user($id);  //Line 45
}

public static function get_file_by_user($id = 0){
   return self::find_by_field("user_id",$id); //Line 164
}

---

Working example :

class A {
    static public function foo() {
        var_dump(static::$bar);
    }
}
class B extends A {
    static protected $bar = "12";

    static public function zoo(){
        self::foo();
    }
}

B::zoo();

Not working example :

class A {
    static public function foo() {
        var_dump(static::$bar);
    }
}
class B extends A {
    static protected $bar = "12";

    static public function zoo(){
        A::foo();
    }
}

B::zoo();