网页刷新而不更改iframe src

I have an iframe on my webpage:

<iframe name="crm_frame" id="crm_frame" src="<?php echo $iframe_src; ?>" width="100%" frameborder="0"></iframe>

and i have tried to add a session variable for the requested_uri

session_start();
if($_SERVER['REQUEST_URI'] == 'index.php')
{
    $_SESSION["iframe_page"] = 'dashboard.php';
}
else
{
    $_SESSION["iframe_page"] = $_SERVER['REQUEST_URI'];
}

and then set the $iframe_src variable for to echo in the SRC of the iframe code

if($_SESSION["iframe_page"] == '')
{
    $iframe_src = 'dashboard.php';
}
else
{
    $iframe_src = $_SESSION["iframe_page"];
}

but its not saving the sesison variable.

any ideas?