<?php
function calculohoras($hora_inicio, $hora_final){
  $datetime1 = new DateTime($hora_inicio);
  $datetime2 = new DateTime($hora_final);
  $interval = $datetime1->diff($datetime2);
  return $interval->format('%H:%I');
}

$horas = calculohoras("2019-05-21 08:00:00", "2019-05-21 17:30:00");
$pausa = calculohoras("2019-05-21 13:30:00", "2019-05-21 14:30:00");

$total = $horas - $pausa;

echo $total;
?>

Notice: A non well formed numeric value encountered