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Get div control by name from codebehind
string divHtml = "";
        for (int j = 1; j < 4; j++)
        {
            string ph = "placeHolder" + j;

            Control phd = (Control)FindControl(ph);
            StringWriter sw = new StringWriter();
            HtmlTextWriter w = new HtmlTextWriter(sw);
            phd.RenderControl(w);
            divHtml = divHtml + sw.GetStringBuilder().ToString();
        }

<div runat="server" id="placeHolder1" class="placeholder" >
</div>

StringWriter sw = new StringWriter();
HtmlTextWriter w = new HtmlTextWriter(sw);
placeHolder1.RenderControl(w);
divHtml = divHtml + sw.GetStringBuilder().ToString();

for (int j = 1; j < 4; j++)
        {
            string ph = "placeHolder" + j;

            ContentPlaceHolder cph = this.Master.FindControl("MainContent") as ContentPlaceHolder;
            Control ctrlPh = cph.FindControl(ph);

            StringWriter sw = new StringWriter();
            HtmlTextWriter w = new HtmlTextWriter(sw);
            ctrlPh.RenderControl(w);
            divHtml = divHtml + sw.GetStringBuilder().ToString();

        }