Usar uma classe dentro de outra

class systemDatabaseMySql {

private $stHost = "127.0.0.1";
private $stUser = "root";
private $stPass = "********";
private $stName = "*****************************";
private $stMySqlConnection = FALSE;
public $stMySqlTypeSelect = NULL;
public $stMySqlTables = NULL;


public function __construct(){
$this->stMySqlConnection = mysqli_connect($this->stHost, $this->stUser, $this->stPass, $this->stName) or die (mysqli_error($this->stMySqlConnection));
}

public function __destruct() {
if($this->stMySqlConnection != FALSE):
mysqli_close($this->stMySqlConnection);
endif;
}

public function systemMySqlQuery($a, $b = NULL){
if($a != NULL):
$stMySqlQuery = $this->stMySqlConnection->query($a);
if($b == "types"):
         $this->stMySqlTypeSelect = $stMySqlQuery;
endif;
endif;
}

public function systemMySqlSelectType($a = NULL){
switch($a):
case("rows") : return mysqli_num_rows($this->stMySqlTypeSelect); break;
case("array") : return mysqli_fetch_array($this->stMySqlTypeSelect); break;
case("assoc") : return mysqli_fetch_assoc($this->stMySqlTypeSelect); break;
case("object") : return mysqli_fetch_object($this->stMySqlTypeSelect); break;
default : return mysqli_fetch_object($this->stMySqlTypeSelect); break;
endswitch;
}

public function systemMySqlInsertData($a)
{
$x = array();
$y = array();

for($i = 0; $i < count($a); $i++):
foreach($a[$i] as $camp => $value):
         eval("\$x[" . $i . "] = '" . $camp . "'; \$y[" . $i ."] = '" . $value . "';");
endforeach;
endfor;

$stMySqlInsertData = "INSERT INTO `" . $this->stMySqlTables . "` (";

for($i = 0; $i < count($y); $i++):
$stMySqlInsertData .= is_numeric($y[$i]) ? $y[$i] : "'" . $y[$i] . "'";
if($i < count($y) - 1):
         $stMySqlInsertData .= ',';
else:
         $stMySqlInsertData .= ");";
endif;
endfor;

return $this->systemMySqlQuery($stMySqlInsertData);
}

public function systemMySqlUpdateData($a, $b, $c){
$x = array();
$y = array();

for($i = 0; $i < count($a); $i++):
foreach($a[$i] as $camp => $value):
         eval("\$x[" . $i . "] = '" . $camp . "'; \$y[" . $i ."] = '" . $value . "';");
endforeach;
endfor;

$stMySqlUpdateData = "UPDATE `" . $this->stMySqlTables . "` SET ";

for($i = 0; $i < count($y); $i++):
$stMySqlInsertData .= is_numeric($y[$i]) ? $y[$i] : "'" . $y[$i] . "'";
if($i < count($y) - 1):
         $stMySqlInsertData .= ',';
endif;
endfor;

$stMySqlUpdateData .= "WHERE `" . $b . "` LIKE '" . $c . "';";

return $this->systemMySqlQuery($stMySqlUpdateData);
}

public function systemMySqlDeleteData($a, $b){
$stMySqlDeleteData = "DELETE FROM `" . $this->stMySqlTables . "` WHERE `" . $a . "` LIKE '" . $b . "'";

return $this->systemMySqlQuery($stMySqlDeleteData);
}

public function systemMySqlSelectData($a, $b){
$stMySqlSelectData = "SELECT * FROM `" . $this->stMySqlTables . "`";

if($a != NULL):
$stMySqlSelectData .= "WHERE " . $a;
elseif($b != NULL):
$stMySqlSelectData .= " " . $b;
endif;

return $this->systemMySqlQuery($stMySqlSelectData, "types");
}
}

class systemComments extends systemDatabaseMySql {

/* Show comments */
public function show (){
$x = new systemDatabaseMySql();

$y = "ORDER BY `commentId` DESC"
$x->stMySqlTables = "system_tbl_accounts_users";
$x->systemMySqlSelectData(null, $y);

if($x->systemMySqlSelectType("rows") > 0 ):
         $w = $y->systemMySqlSelectType("object");
         echo($w->commentContent);
else:
         echo("No comments");
endif;
}
}