GTM AMP Container Proxy (for sharing GA Client ID with non-AMP pages) - for uti.edu

GTM_AMP_Container_Proxy.php

<?php

// Callback for the GET request
function gtmAmpProxy_serve(){

	//error_reporting(0);
	
	// Get the hostname of the request origin, and parse it for the pure domain name.
	$domain = explode( ':', $_SERVER['HTTP_HOST'] )[0];
	if( !( $cookieDomain = @$_GET['cookieDomain'] ) ){
		$cookieDomain = str_replace( 'www.', '', strtolower( $domain ) );
		if( substr( $cookieDomain, -8 ) === '.uti.edu' ){
			$cookieDomain = 'uti.edu';
		}
	}

	// Check if the browser already has the _ga cookie. If not, generate a new cookie. 
	$cid = $_COOKIE['_ga'];
	if( !isset( $cid ) ){
		$domainLength = count( explode( '.', $cookieDomain ) );
		$cid          = "GA1.{$domainLength}.".rand( 100000000, 999999999 ).'.'.time();
	}

	// Store the actual Client ID (last two numbers) of the _ga cookie value in the $cidNumber variable 
	$cidNumber = preg_replace( '/^GA.\.[^.]+\./', '', $cid );

	// Fetch the actual GTM container, by passing the valid query parameters from the original request.
	$container = file_get_contents( "https://www.googletagmanager.com/amp.json?$_SERVER[QUERY_STRING]" );

	// Replace the &cid; parameter value with ${clientId}
	$container = preg_replace( '/(&cid=)[^&]+/', '${1}${clientId}', $container );
	
	// Add the clientId to the "vars" object in the container JSON.
	$container                 = json_decode( $container );
	$container->vars->clientId = $cidNumber;

	// Build a new HTTP response from the modified configuration file.
	$response = json_encode( $container );
	
	// Clear output that could create an invalid response (i.e. PHP warnings).
	@ob_clean();

	// Clear any existing response headers.
	header_remove();
	
	// Add the required headers (Set-Cookie, most importantly) to the Request
	http_response_code( 200 );
	header( "Set-Cookie: _ga={$cid}; Domain={$cookieDomain}; Path=/; Secure; SameSite=None; "
	        ."Expires=".date( 'D, j F Y H:i:s', time()+60*60*24*365*2)." GMT;" );
	header( 'Access-Control-Allow-Origin: https://cdn.ampproject.org' );
	header( "AMP-Access-Control-Allow-Source-Origin: https://{$domain}" );
	header( 'Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin' );
	header( 'Content-Type: application/json; charset=utf-8' );

	// Return the HTTP response.
	echo $response;
	exit();
}

gtmAmpProxy_serve();