How to make equatition between a laravel helper variable and php variable in blade

Hi everyone i have a laravel project and for my category detail page i have to make equatation.

//THIS IS MY CATEGORYDETAIL.BLADE PAGE
<div class="categories_product_area">
<div class="row">
 = $CategoryId 
   <?php 
   $ProductListByCategoryIds = \DB::table('products')
  ->orderBy('id', 'desc')
   //->where('featured',  1)
   ->where('product_category', '=', $CategoryId)
   //->limit('1')
   ->get();
   ?>

i'm getting $CategoryId isn't defined error. i'm beginner. please excuse my mistakes

via Chebli Mohamed