Re: how to put document.getElementById("sidebar").innerHTML ETC into an ECHO

That is because you need to remove the var...

it should be:
document.getElementById("sidebar").innerHTML = "sidebar_html";

Although, what you probably want is (assuming that you want the html
that is assigned to the variable sidebar_html to be displayed and not
the text "sidebar_html"):
document.getElementById("sidebar").innerHTML = sidebar_html;

which means your PHP should be:
echo "document.getElementById(\"sidebar\").innerHTML = sidebar_html\n";

Have you posted a link to the problem page somewhere?

-- Larry