Removed PHP warnings with E_ALL level

htdocs/admin/syslog.php

htdocs/core/events.class.php

htdocs/html.formadmin.class.php

htdocs/includes/menus/barre_left/eldy_backoffice.php

htdocs/index.php

htdocs/interfaces.class.php

htdocs/lib/admin.lib.php

htdocs/lib/functions.lib.php

htdocs/lib/security.lib.php

htdocs/main.inc.php

htdocs/master.inc.php

htdocs/product/index.php

htdocs/product/pre.inc.php

htdocs/user.class.php