why is there a $ in php code example

Example: ?? ' ' in php

// Null Coalesce Operator - $statement ?? ''
$categoryName = $category->name ?? 'Not Available';

// The above Statement is identical to this if/else statement
if (isset($category->name)) {
    $categoryName = $category->name;
} else {
    $categoryName  = 'Not Available';
}