Ffmpeg code to normalize 3 mp3s together

normalize.php

<?php
require 'vendor/autoload.php';
$opener="intro.mp3";
$closer="outro.mp3";
$audioFile = "final.mp3";

$bookSummaryFile="summary.mp3";
$ffprobe = FFMpeg\FFProbe::create();
$openerDuration = $ffprobe->format($opener)->get('duration');
$summaryDuration = $ffprobe->format($bookSummaryFile)->get('duration');
$closerDuration = $ffprobe->format($closer)->get('duration');
$summaryTransition = 5;
$closerTransition = 4;
$totalDuration = $openerDuration + $summaryDuration + $closerDuration -$summaryTransition - $closerTransition;
$summaryDelay =round($openerDuration - $summaryTransition);
$closerDelay = round($summaryDuration + $openerDuration - $summaryTransition - $closerTransition);
$audioFile = DIR . '/mp3/merged.mp3';

$filter="[1]adelay={$summaryDelay}s[b];[2]adelay={$closerDelay}s[c];[0][b][c]amix=inputs=3:dropout_transition={$totalDuration},volume=3";
$dynaudnorm=",dynaudnorm=f=200:g=15";
$acompressor=",acompressor=level_in=2:threshold=10^(-30/20):attack=200:release=1000:detection=0:makeup=10^(5.1/20)";

exec("ffmpeg -i {$opener} -i {$bookSummaryFile} -i {$closer} -filter_complex \"{$filter}{$acompressor}\" {$audioFile}")

?>