Fix line-numbers for PHP in markup

The problem is, that inline php code can be anywhere inside of a html
element. To properly highlight this the php language implementation
temporarily replaces all inline php code in `env.code` with something
like `{{PHP0}}`. After the markup highlighting is finished it replaces
it again with the real code, but it leaves `env.code` corrupted.
But the line-numbers plugin uses `env.code` to count the
lines. Additionally, depending on the order the hooks are executed, the
php implementation replaces the entire code with `env.element.innerHTML
= env.highlightedCode;`, thereby deleting the line number side bar
previously added by the line-numbers plugin.

So to fix this, it must be ensured, that `env.code` is restored before
the line-numbers plugin uses it and the line numbers side bar, if
present, must be saved when `env.element.innerHtml` is replaced.