As far as it is concerned running PHP projects under IIS, it is possible to achieve, here is an answer on SO that might help in this regard.
But if you want to mix up ASP.NET C# and PHP code then I suppose it is not possible. The real question is, why on earth would you want to go this way? Is there something PHP can do and C# cannot? To me, C# seems way more elegant than PHP, why would you want to do it anyway?
As far as it is concerned being able to echo some data on front-end you can do that in C# like this (or using many other ways that might suit your scenario):
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="MeineWebseite.löschen.WebForm1" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
</head>
<body>
<form id="form1" runat="server">
<asp:Literal runat="server" ID="ltEchos"/> <!-- Would share a sample to use this control to populate things -->
</form>
</body>
</html>
.CS Code Behind:
public void EchoSomeDataWithResponseWrite()
{
Response.Write("Hallo \"Welt\""); // echo, that does not involves the Literal control
// to give a line break, put your html like below
Response.Write("<br />");
var name = "John Doe"; // to write name
Response.Write(name);
// to write some raw script:
Response.Write("<script>alert('Weather is fine today!')</script>");
}
public void EchoSomeDataWithLiteralControl()
{
var name = "John Doe";
var html = "Hallo \"Welt\" <br /> ";
html += name + "<br />";
ltEchose.Text = html;
}
Power is in your hand, you can achieve anything using C# that you could've achieved using PHP.