JSON encoded array comes out as null in PHP-Collection of common programming errors

I am making a small drag and drop app using jQuery UI’s Sortable feature. The jQuery works fine but when I try to encode the array of all of the lines in their order, the PHP script on the other side reads the array as NULL and I cannot figure out why. Here’s everything in the order that they appear in my first script:

The HTML

    
    REORDER

The JS

$(document).ready( function(){

    var formOrder = document.getElementById('reorder');
    formOrder.addEventListener('submit', function(){
        var post_ids_field= document.getElementById('post_ids');
        var ul_id = document.getElementById("photo-selector");
        var post_ids = ul_id.getElementsByTagName("li");
        post_ids_field.value = JSON.stringify(post_ids);
        alert("check");
    });

});

When clicking the “REORDER” button in the form up top it sends to orderupdate.php which is as follows