Question 101426 in PHP Question posted in
Official manual can be found here.

Cannot call static function in class from another non-static class in PHP

I am now trying this out for more 12 hours and am really frustrated cause it does not work.

I have two classes Book and Comment. Each Book has multiple Comments. I want to return a specific book with its n-Comments. For this, i created both classes, cause each class should also have additional operations like Add, Delete, Update etc.

My problem is, when i call a static function in Comment class from a non-static function from the book class it does not work:

 $result = Comment::GetCommentByBookId($id);

Curious thing is, when i change the static function GetCommentByBookId in the Comment class to a non-static function, and change the line in the GetBookById function to

$c = new comment;
$result = $c->getCommentByBookId(1);

it works! But why does it not work when i try to call static function?

These are the classes:

Book

<?php
    require_once ("class/DBController.php");
    require_once ("class/Comment.php");

    class Book {
        private $db_handle;

        function __construct() {
            $this->db_handle = new DBController();
        }
        
        function GetBookById($id) {
            $query = "SELECT * FROM book WHERE id = ?";
            $paramType = "i";
            $paramValue = array(
                $id
            );
            
            $c = new comment;
            $result = $c->getCommentByBookId(1);
            //$result = Comment::GetCommentByBookId($id);
            return $result;
        }

        function GetAllBooks() {
            $query = "SELECT * FROM book";
            $result = $this->db_handle->runBaseQuery($query);
            return $result;
        }
    }
?>

Comment

<?php
    require_once ("class/DBController.php");

    class Comment {
        private $db_handle;

        function __construct() {
            $this->db_handle = new DBController();
        }
        
        static function GetCommentByBookId($id) {
            echo "Entered 'GetCommentByBookId'<br>";
            $query = "SELECT * FROM comment WHERE book_id = ?";
            $paramType = "i";
            $paramValue = array(
                $id
            );
            
            echo "Pre-Ex 'GetCommentByBookId'<br>";
            
            $result = $this->db_handle->runQuery($query, $paramType, $paramValue);
            
            echo "Executed successfully 'GetCommentByBookId'<br>";

            return $result;
        }
    }
?>
Tags:

2

Answers


  1. 0

    If you don’t instantiate a Comment instance, you can’t access $this->db_handle.

    You can consider doing like below:

    static function GetCommentByBookId($id) {
     echo "Entered 'GetCommentByBookId'<br>";
     $query = "SELECT * FROM comment WHERE book_id = ?";
     $paramType = "i";
     $paramValue = array(
        $id
     );
            
     echo "Pre-Ex 'GetCommentByBookId'<br>";
            
     $result = (new DBController())->runQuery($query, $paramType, $paramValue);
            
    echo "Executed successfully 'GetCommentByBookId'<br>";

    return $result;
}
    Login or Signup to reply.
  2. 0

    Question: But why does it not work when i try to call static function?

    Answer: Because when you call a static method outside the class then construct of that class does not run and hence if you are calling any other property which is dependent on construct class you will get the error.

    In your case when you called static function GetCommentByBookId($id) in that static method you are calling $result = $this->db_handle->runQuery($query, $paramType, $paramValue); now as construct does not ran hence this->handle did not instanciated the class when you called static method hence you get the error for $this->db_handle->runQuery

    **Example**

    I have reconstructed your code you explain what i mean. Please have a look at the code and the result image.

    <?php 
error_reporting(E_ERROR |  E_PARSE);
ini_set('display_errors', '1');


 class Comment {


         function __construct() {
            $this->dbconnection = new DBController();
            echo "<br>construct was invoked and this msg is from construct of the comment class <br>";
            
        }
        
        //static way
         static function GetCommentByBookId($id) {
         $result = new DBController();
         $result = $result->runQuery('query here');

           return $result."here are the comments for Book ID - " . $id;
        }

        //normal way
        function GetCommentByBookId2($id) {
        $result = $this->dbconnection;
         $result = $result->runQuery('query here');

           return $result."here are the comments for Book ID - " . $id;
        }


    }





class DBController{

    function runQuery(){
        return "database connected and query was executed <br>";
    }
}




class Book {
             
        //this function uses normal method in comment class
        function GetBookById($id) {
            $c = new comment;
            $result = $c->GetCommentByBookId2(1);
            // $result = Comment::GetCommentByBookId($id);
            return $result;
        }

         //this function uses static method in comment class. No class instanciated. 
        function GetBookById2($id) {
            
            $result = Comment::GetCommentByBookId($id);
            return $result;
        }

       
    }


//using classes.


echo '<hr> <strong>Without Static Method</strong> <br> Here we called normal public method defined in the comments class. Here constructor is invoked<br><br><br>';


$bookinfo = new Book;
echo $bookinfo->GetBookById(2) ;

echo '<hr> <strong>Static Method</strong> <br> Here we called the static method defined in the comments class. We get the result because we are instanciating the DB class in static method. In static Construct did not run<br><br>';
echo $bookinfo->GetBookById2(3) ;
 ?>

    Result:
    enter image description here

    If you have any query/issue or find something incorrect please comment.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search