Queries to get unpaid and unpaid invoices

There seems to be a section in your code that you repeat three times. This can probably be placed in a separate function. Something like this:

function getInvoiceTotal($status, $onlyOverdue = FALSE)
{
    $query = Capsule::table('tblinvoices');
    $query->select(Capsule::raw('SUM(total) as total'))
          ->where(['userid' => $_SESSION['uid'], 
                   'status' => $status]);
    if ($onlyOverdue && ($status == 'Unpaid')) {
        $query->whereDate('duedate', '<=', Carbon::now()->format('Y-m-d'));
    }    
    return $query->get()->total;        
}


add_hook('ClientAreaPageInvoices', 1, function($vars) {
    return ['totalAberto'  => getInvoiceTotal('Unpaid'), 
            'totalVencido' => getInvoiceTotal('Unpaid', TRUE), 
            'totalPago'    => getInvoiceTotal('Paid')];
});

Regretably I don't have Laravel installed so I cannot test this code, but I think the general idea is clear. Perhaps someone with more knowledge of Laravel can add to this answer?

Some notes:

• I changed array(....) to [....].
• I didn't understand the json_decode(json_encode()) bit. Perhaps to go from objects to arrays? I don't see what's wrong with using the actual objects. Here's where I have most doubt that my code is correct, since I haven't tested it.
• I left out a lot of variables you used. Using variables, with good names, can make code easier to read, but in really good code, when a variable is only used once, it shouldn't be necessary.
• I strongly advise you to write your code in English only. I had to find out what the meaning of 'Aberto', 'Vencido' and 'Pago' is. Not every programmer speaks Protuguese.

SUM aggregates

Instead of selecting the SUM aggregate manually:

->select(Capsule::raw('SUM(total) as total_aberto'))

The Builder::sum() aggregate method can be used:

->sum('total')

Is there a model defined like Invoice?

<?php

namespace App\Models;

class Invoice extends Model 
{

    protected $table = 'tblinvoices';

}

Presuming such a model existed, the code like this:

$query_1 = Capsule::table('tblinvoices')
    ->select(Capsule::raw('SUM(total) as total_aberto'))
    ->where(array('userid' => $user_id, 'status' => 'Unpaid'))
    ->get();

could be simplified to this, using the methods to retrieve Single Models / Aggregates:

$totalAberto = App\Models\Invoice::where(['userid' => $user_id, 'status' => 'Unpaid'])
                                  ->sum('total');
    

Array syntax

There isn't anything wrong with using array() but as of the time of writing, PHP has Active support for versions 8.0 and 7.4, and since PHP 5.4 arrays can be declared with short array syntax (PHP 5.4), as was used in my sample at the end of the section above.