调用类方法时出现“调用未定义的函数”错误

这是错误
，这是代码，正如你所看到的，我显然已经定义了函数，所以为什么它不起作用Fatal error: Call to undefined function assign(

class shades {
    function create($name, $shades, $slug, $shortDesc, $longDesc, $position){
        $name = sanitize_paranoid_string($name);
        $slug = slug($name);
        $shortDesc = sanitize_sql_string($shortDesc);
        $longDesc = sanitize_sql_string($longDesc);
        $query = mysql_query("INSERT INTO products (type, name, slug, shortDesc, htmlDesc, position)VALUES('shades','$name','$slug','$shortDesc','$longDesc','$position')")or die(mysql_error());  
        $ID = mysql_insert_id();
        assign($shades, $ID);
        if($query) {return true;}
        else {return false;};
    }
    function delassign($toID){
        mysql_query("DELETE FROM assign WHERE type='shades' AND toID='$toID'")or die(mysql_error());    
    }
    function assign($shades, $toID)
    {
        foreach($shades as $shade)
        {
            $result = mysql_query("INSERT INTO assign(type, typeID, toID)VALUES('shades','$shade','$toID')")or die(mysql_error());
            if($result){echo "Added!";}
            else{echo"Not Added!";}
        };  
    }
}