Select in two different tables

Good Night;

I have two tables created in the same database. For both tables, I have two selects. The return of a select would like to concatenate the second select to bring the desired result and save to a variable via PHP.

First select:

    $opcoes3 = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES UTF8');
   $conexao = new PDO("mysql:host=".SERVER."; dbname=".DBNAME, USER, 
   PASSWORD, $opcoes3);
      $sql3 = "SELECT * FROM receita WHERE datareceita = (SELECT 
      MAX(datareceita) FROM receita)";
      $stm3 = $conexao->prepare($sql3);
      $stm3->execute();
       while($row3 = $stm3->fetch())  {
       $receitatotal = $row3['receita'];
      }

The above select brings the result of a value that I set as an example in the database: 11400

The second Select:

<?php
    $anomes2 = date('y/m');
    $anomes2 = str_replace("/", "", $anomes2);
    $opcoes2 = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES UTF8');
    $conexao2 = new PDO("mysql:host=".SERVER."; dbname=".DBNAME, USER, 
    PASSWORD, $opcoes2);
    $sql2 = " SELECT (SUM(-valor_boleto)+".$receitatotal.") as total, extract(year_month 
   from vencimento) as periodo FROM boleto
   GROUP BY extract(year_month from vencimento)= '"."20".$anomes2."'";
    $stm2 = $conexao2->prepare($sql2);
    $stm2->execute();

    while($row2 = $stm2->fetch())  {
    $soma = $row2['total'];
     }

         ?>

The $ receitatotal I concatenated in the second select to generate a sum, the value of the $ receitatotal is the 11400. But when it sums, it returns a value totally than expected.