Setting an If Statement with AJAX in PHP

I am nearing completion of this code / plugin and I just have one bit to finish up before we make it all pretty and presentable; I am attempting to get my PHP file to re-load/re-fresh with the POST variables sent from the AJAX file, all from the response of the AJAX send:

$("#CategoryTree").load("poster.php #CategoryTree"); 

Right now on clicking a list item the CSS fires, the ajax fires and responds, variables are sent (checked them in the action registry as well all good there) but the area I call to re-load with the poster.php #CategoryTree does not react, error 500 is thrown.

Here is my full Javascript code:

jQuery(document).ready(function($) {     $('#CategoryTree').on('click', 'input[type=checkbox]', function() {         var $this = $(this);         var data = '';         if ($this.is(":checked")) {             $this.addClass('selectCheckbox');             //$this.val("1");             data = {                 action: 'catID_Callback',                 catID: $this.attr('id'),                 catState: 1                 };             $.post(the_ajax_script.ajaxurl, data, function(response) {                 //alert('Got this from the server: ' + response);                 $("#CategoryTree").load("poster.php #CategoryTree");                                 //alert( "Load was performed." );});                  console.log(response);             });         } else {             $this.removeClass('selectCheckbox');             //$this.val("0");             data = {                 action: 'catID_Callback',                 catID: $this.attr('id'),                 catState: 0                 };             $.post(the_ajax_script.ajaxurl, data, function(response) {                 // alert('Got this from the server: ' + response);                 console.log(response);             });          }     });  }); 

And here is the PHP code (Line 55 is where the check starts):

alert('$message');";  $thearray = []; $terms = get_terms("pa_mymelp"); foreach ( $terms as $term ) { $categories =  $term->name; array_push($thearray, $categories); }  $categoryLines = $thearray;   function buildCategoryTree($categoryLines, $separator) {     $catTree = array();     foreach ($categoryLines as $catLine) {        $path = explode($separator, $catLine);        $node = & $catTree;        foreach ($path as $cat) {            $cat = trim($cat);            if (!isset($node[$cat])) {                $node[$cat] = array();            }            $node = & $node[$cat];        }     }     return $catTree; }  function displayCategoryTree($categoryTree, $indent = '') {     foreach ($categoryTree as $node => $children) {         echo $indent . $node . "
";         displayCategoryTree($children, $indent . '|- ');     } }  $categoryTree = buildCategoryTree($categoryLines, '/');  function displayHtmlCategoryTree($categoryTree, $id = null, $pathSeparator = '/', $parents = '') {     if (empty($categoryTree)) return '';      $str = '';     foreach ($categoryTree as $node => $children) {         $currentPath = $parents . (empty($parents) ? '' : $pathSeparator) . $node;         $thelink = '';         $opener = 0;         if (substr_count($currentPath, '/')==5){         $patterns = array(" ", "/", ".");         $thelink = 'http://caap.co.nz/?pa_mymelp=' . strtolower(str_replace($patterns, '-', $currentPath));         $str .= '
' . '' . '' . '' . $node . '' .          /*displayHtmlCategoryTree($children, null, $pathSeparator, $currentPath) .          */'
';}         else                 {         $cat = 0;         $catState = 0;          if (isset($_POST['catID'])){             $cat = $_POST['catID'];         }          if (isset($_POST['catState'])){             $catState = $_POST['catState'];         }          $str .= '
' . '' . '' . $node . '';             if ($cat == $currentPath && $catState == 1 ){$str.=displayHtmlCategoryTree($children, null, $pathSeparator, $currentPath);}           }           }      $str .= '
';     return $str; }     echo displayHtmlCategoryTree($categoryTree, "CategoryTree", '/'); /* function add_query_vars_filter( $vars ){   foreach (array_keys(Contracts::$query_params) as $name)     $vars[] = $name;   return $vars; } add_filter( 'query_vars', 'add_query_vars_filter' ); */ ?> 

Here is the pertinent code from the PHP file

        $cat = 0;         $catState = 0;          if (isset($_POST['catID'])){             $cat = $_POST['catID'];         }          if (isset($_POST['catState'])){             $catState = $_POST['catState'];         }          $str .= '
' . '' . '' . $node . '';             if ($cat == $currentPath && $catState == 1 ){$str.=displayHtmlCategoryTree($children, null, $pathSeparator, $currentPath);}