Pull A Value Being Stored In A Php Function

I am trying to pull a value being stored in a php function (it works fine) which is echoed into an html element, and use it in a javascript function. my HTML:

<img id="menu_image" src="<?php echo get_php_value(); ?>" width="356" />

my javascript:

function get_large_image(){
var image = document.getElementById('menu_image').src;
alert(image);
}

right now the alert is giving me flie path to the current page, I need a file path to the image. When I had javascript pull a value from an input tag using the id to reference and stored the php function in the value attribute, I got this: "object HtmlElement" The final purpose of all this is to modify dynamically and HTML <a>'s href and class attribute based on a certain condition being true.