Is it correct to merge audio files using fwrite() in PHP?

Question

I’m developing a script of a captcha simple in PHP and I challenged myself to be able to generate an audio file to help people with visual impairments.

I didn’t know exactly how to do this, but I created an audio file for each letter of the alphabet and called it 'letter name. mp3 (example: w.mp3'), and, in an archive PHP called 'audio php.', designed to create the final audio file, I took the letters from the captcha and added the audio of each letter using the functions file_get_contents() and fwrite(), native to the PHP; to my surprise, it worked exactly as I wanted it.

My question is, although it worked, is it correct to do so? Reading the contents of one file and writing it in another is simple in text files and similar, but in more complex files like audios, I have my doubts if it is ideal. If yes, it is recommended?

audio php.:

<?php
    session_start(); //início da sessão
    $vetorLetras = str_split($_SESSION["captcha"]); //código do CAPTCHA
    $arquivo = "audios".DIRECTORY_SEPARATOR.session_id().".mp3"; //nome do arquivo a ser criado
    $f = fopen($arquivo, "a+"); //arquivo é criado
    for ($i = 0; $i < 5; $i++) {
        //a cada repetição o arquivo recebe o conteúdo do áudio de uma letra
        $x = file_get_contents("audios".DIRECTORY_SEPARATOR.$vetorLetras[$i].".mp3");
        fwrite($f, $x);
    }
    fclose($f); //arquivo é fechado
    //força-se o download do arquivo
    header("Content-Type: application/octet-stream");
    header("Content-Transfer-Encoding: binary");
    header("Content-disposition: attachment; filename=\"audio.mp3\"");
    readfile($arquivo);
    //arquivo é excluído
    unlink($arquivo);
?>

Source code