致命错误:不在对象上下文中使用$this不能正常工作

<?php
class Functions {
    public static function extendSql($dbhost, $dbuser, $dbpass, $dbname) {
        // making the mysql connection dynamically editable
        $mysql_connect  =   mysql_connect($dbhost, $dbuser, $dbpass)or die("Could not connect: " . mysql_error());
        $mysql_select_db =  mysql_select_db($dbname) or die(mysql_error());
    }
    public static function whileLoop($dbuser, $dbpass, $dbname, $sql, $passedData) {
        $this->extendSql($dbuser, $dbpass, $dbname);
        $results = mysql_query($sql);       
        while ($row = mysql_fetch_array($results)) {
        echo $passedData;
        }
    }
}
Functions::whileLoop("root", "", "rand", "SELECT * FROM products",
$hello = "hi all");
?>

致命错误:当不在对象上下文中使用$this时C:'Workspace'htdocs'Misc-2'nurbell1'core'conf'misc.php on line 13

我做错了什么?显然，在我的代码中，$this是在类的上下文中引用的。