Help me PHP 2 errors how to fix

PHP Notice: Undefined variable: _SESSION in /captcha/captcha.php on line 29

PHP Fatal error: Call to undefined function imageantialias() in /captcha/captcha.php on line 75


Собственно
29 строка

29 if( !$_SESSION )
30 session_start();
31 $_SESSION['captcha_key'] = $captcha;

Code

29   if( !$_SESSION )

30        session_start();

31   $_SESSION['captcha_key'] = $captcha;

и 75 строка

72 $n = rand(0,sizeof($font_arr)-1);
73 $img_fn = $img_arr[rand(0, count($img_arr)-1)];
74 $im = imagecreatefrompng (IMG_DIR . $img_fn);
75 imageantialias($im, true);

Code

72     $n = rand(0,sizeof($font_arr)-1);

73        $img_fn = $img_arr[rand(0, count($img_arr)-1)];

74         $im = imagecreatefrompng (IMG_DIR . $img_fn);

75        imageantialias($im, true);