class mysql_connector{

      function db_connect($server, $user, $pass, $base)
      {
        mysql_connect($server,$user,$pass) or die("Error to connect to db");
        mysql_select_db($base) or die("couldnt select db");  
      }
      
      function get_content_by_id($id)
      {
        $sql = "SELECT * FROM `content` WHERE `id` = ".$id;
        $sql_result = $this->mysql_qt($sql); //Вот так не работает
      return $sql_result['content'];
      }
      
      
      function get_title_by_id($id)
      {
      $sql = "SELECT * FROM `content` WHERE `id` = ".$id;
      $sql_result = mysql_connector::mysql_qt($sql); //Вот так работает
      return $sql_result['caption'];
      }

       public static function mysql_qt($query)
      {
       $sql = mysql_query($query);
        if(mysql_num_rows($sql)==0){
            return 0;
        } else {
        $sql_result = mysql_fetch_assoc($sql);
        return $sql_result;
        }
      }
  }

<?php
class foo{
  function weather($date){
    $temperature = $date;
    return $temperature;
  }

  function call(){
      $date = "123";
      $r = $this->weather($date);
      return $r;
  }
}

$obj = new foo;
echo $obj->call();