PHP OO Call to a non-object [duplicate]

This question already has answers here: Call 开发者_如何转开发to a member function on a non-object (4 answers) Closed 10 years ago.

I have recently started coding in php again and am learning Object Orientated PHP. I have come across an issue that I cannot seem to get my head round with an error:

Fatal error: Call to a member function verify_Username_And_Password() on a non-object in C:\wamp\www\sso\lib\UserManagement.php on line 57

I understand that the program seems to think that the mysql class variable is empty but not why when it is instanced.

The function verify_Username_And_Password ($username, $password) is defined in the class Mysql.

The following is code from UserManagement.php with parts taken out that are not needed.

<?PHP
require_once 'lib/Mysql.php';

class UserManagement {
    private $mysql;

    function __construct(){
        session_start();
        $this->manage_Session();
        $this->mysql = new Mysql();
    }

    function validate_User ($username, $password){
        $user_Check = $this->mysql->verify_Username_And_Password($username, md5($password));

        if($user_Check) {
            return true;
        } else {
            return false;
        }
    }
}

I must be missing something basic about OO in PHP. I would be grateful for any advice anyone can give. I will be happy to post more code if needed.

As requested here is: Mysql.php pastebin. UserManagement.php pastebin

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Without seeing the code that does the work, my guess would be you are using the validate_User method without first constructing a UserMangement object.

$um = new UserManagement();
$um->validate_User($username, $password);

If you wish to call validate_User without first constructing a UserManagement object it needs to be a static method. Without the UserManagement object being constructed Mysql is never constructed and saved in the attribute inside your class.