It's probably a scope issue? You are assigning $send_email in one function and trying to use it in another?
function assign_send_email($email)
{
$send_email = $email;
}
// This won't work as expected
echo $send_email;
Make it a global
function assign_send_email($email)
{
global $send_email;
$send_email = $email;
}
// Now it works
echo $send_email;