How to validate authentication header in multiple functions in PHP - php

Question

I'm trying to use JWT to create a authentication system using AngularJs and PHP. I understood the concept of JWT, I can modify the headers and send my access token, etc... My doubt is on the logic to elaborate this since I have a lot of different functions.
For example, I have a file called client.php where I have client related functions, such as getClient, updateClient, insertClient, etc.. The same applies for other files, product.php, category.php, filter.php and so on.
So, how am i supposed to do this token validation before actually running any of these functions?
I tought of creating a common function and use it inside each function, for example:
function.php
function checkToken($token) {
$serverToken = //getting server token;
if($token !== $serverToken) {
return false;
}
}
client.php
require_once 'function.php';
$tokenError = array('status' => 'Unauthorized');
function insertClient() {
global $tokenError;
$accessToken = $_SERVER['HTTP_TOKEN'];
if(!checkToken($accessToken)) {
return $tokenError;
} else {
//Proceed with the function
}
}
function getClient() {
//Repeat samething as insertClient
//...
}
But it doesn't feel like the proper way to do this. So, is there a better, or a correct way, to do headers validation/authorization on each http request the user make?