Reliably check for the checkout page earlier than is_checkout()

The is_checkout() method is the standard to build conditionally logic around on whether the user is on the checkout page or not.

But what if it is too early for is_checkout() to be available?

Paying attention to the WordPress load lifecycle, is_checkout is available “too late” in the cycle.

It may work when called in a function hooked to template_reditect or wp_head.

If you need it before the template has loaded, is_checkout renders completely useless.

So, you need to get creative to check whether the current page is the checkout page or not.

A solution I came up with recently for a project is like so:

add_action('wp_loaded', function(){
  
    //Get the page ID from the WooCommerce option.
    $checkout_page_id = get_option( 'woocommerce_checkout_page_id' );

    // Transform the obtained string value into an integer.
    $checkout_page_id = (int)$checkout_page_id;

    // Get the Request Slug.
	  $request_uri = $_SERVER['REQUEST_URI'];
	  // Get the Post ID from the obtained slug.
	  $post_id = url_to_postid($request_uri);

    // Compare the two IDs.
	  if( $post_id == $checkout_page_id ) {
		    exit('this is the checkout page'); // If the IDs match identically, then we are on the checkout page.
	  }
});

The example above uses a combination of retrieving the checkout page ID stored in the wp_options table as a WooCommerce option.

Then obtains the Page ID of the current query.

When the IDs match, we are on the checkout page.