500エラーについて

はじめまして。初めて質問させて頂きます。
現在、php初心者の自分が何とかWordpressでサイト構築を行っています。
ほぼ、完成した所で突然の５００エラーで途方に暮れています。

自力で解決しようと、色々ファイルをいじりすぎて記憶が混乱していますが
確か、wp-settings.php　にWP_MEMORY_LIMIT　の設定を追加しようとしてから
おかしくなったような・・・

そんな中、デバックモードの存在を知りエラー内容は表示されたのですが
初心者の自分には解読不能な内容でしたので、皆様のお知恵をお借りしたく
書き込みさせて頂きます。

問題のエラー内容は下記の通りです。
Notice: Constant EZSQL_VERSION already defined in /var/www/html/wp-content/db.php on line 43

Notice: Constant object already defined in /var/www/html/wp-content/db.php on line 48

Notice: Constant OBJECT_K already defined in /var/www/html/wp-content/db.php on line 53

Notice: Constant ARRAY_A already defined in /var/www/html/wp-content/db.php on line 58

Notice: Constant ARRAY_N already defined in /var/www/html/wp-content/db.php on line 63

Notice: Constant EZSQL_VERSION already defined in /var/www/html/wp-content/plugins/db-cache-reloaded-fix/db-module.php on line 43

Notice: Constant object already defined in /var/www/html/wp-content/plugins/db-cache-reloaded-fix/db-module.php on line 48

Notice: Constant OBJECT_K already defined in /var/www/html/wp-content/plugins/db-cache-reloaded-fix/db-module.php on line 53

Notice: Constant ARRAY_A already defined in /var/www/html/wp-content/plugins/db-cache-reloaded-fix/db-module.php on line 58

Notice: Constant ARRAY_N already defined in /var/www/html/wp-content/plugins/db-cache-reloaded-fix/db-module.php on line 63

Notice: load_plugin_textdomain was called with an argument that is deprecated since version 2.7 with no alternative available. in /var/www/html/wp-includes/functions.php on line 2925

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID ‘UTC-9’ is invalid in /var/www/html/wp-content/plugins/event-calendar/tz.php on line 27

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID ‘UTC-9’ is invalid in /var/www/html/wp-content/plugins/event-calendar/tz.php on line 27

Notice: Use of undefined constant headerWpToTop – assumed ‘headerWpToTop’ in /var/www/html/wp-content/plugins/wp-to-top/wp-to-top.php on line 241

Notice: Use of undefined constant footerWpToTop – assumed ‘footerWpToTop’ in /var/www/html/wp-content/plugins/wp-to-top/wp-to-top.php on line 243

Fatal error: Call to a member function register_handler() on a non-object in /var/www/html/wp-includes/media.php on line 953

Notice: Use of undefined constant user_level – assumed ‘user_level’ in /var/www/html/wp-content/plugins/ultimate-google-analytics/ultimate_ga.php on line 525

沢山あって大変かと思いますが、どなたか解決策を教えて下さい。

何卒、宜しくお願い致します！