Fixed 'before-insert' hook

Fixed **'before-insert'** hook: now checks if language is PHP before assigning `env.code` to `env.backupCode`.

Only "language-php" defines `env.backupCode`: without this condition, `env.Code` would be undefined for any other language than PHP after this hook is called. It would then lead to wrong behaviours (eg. *Line Numbers* plugin would not show lines).