How to pass JavaScript variable to in php

How to pass JavaScript variable to in php

I want to pass javascript variable as a path to <img src="path"> in php
my code is
javascript code

`
function showSmallImage(path)
{
document.getElementById("small_image").value = path;
}
`i am getting path and i have to pass it to
php code

echo "<img id='small_image' src='".$path."'>";but i am not getting any way
to solve it please suggest me any way to solve this issue.
Thanks in advanced!!!