致命错误：不在对象上下文中时使用$ this

Question

I got this error, i cant figure whats problem. My code is

class Example{
     public function get_lang()
        {
            $jezik = get_option('jezik');
                     switch ($jezik ) {
                         case $jezik == 'rs_RS':
                         $drzava = 'NazivSrb';
                         break;
                         case $jezik == 'ro_RO':
                         $drzava = 'NazivRo';
                         break;
                         case $jezik == 'uk_UK':
                         $drzava = 'NazivRu';
                         break;
                    }
            return $drzava;        
        }

    static public function ExecuteSql($where_criteria=NULL) {
            global $wpdb;

                   $drzva = $this->get_lang();

            $upit = "
                            SELECT ID, Naziv, $drzava, PhoneCode, Kod
                    FROM wp_drzava";
            if ($where_criteria)
                $upit .=" WHERE ". $where_criteria ;
            $upit .= " GROUP BY Kod";
            //echo $upit;
            return $wpdb->get_results($upit);
        }

{

Basically I try to use get_lang method return value, and save it to variable in other method, and pass it to query. But i get an error in this line

$drzva = $this->get_lang();

Answer 1

You can't use $this inside static method.. use self keyword if you want to use same class method inside static method

self::get_lang();

EDIT: get_lang() should static method, if you are not using class properties inside method.

Answer 2

You cannot use $this in a static public function ( $this does not exist in it).

To fix this, use:

$example = new Example;
$drzva = $example->get_lang();