PHP concatenate to variable

I have the following function that sends a message to selected users

function send_msg($user_id, $title, $message){
$args = array( 'recipients' => $user_id, 'sender_id' => bp_loggedin_user_id(), 'subject' => $title, 'content' => $message );
$thread_id = messages_new_message1( $args );
messages_delete_thread($thread_id,bp_loggedin_user_id());
}

This is invoked by the following code that posts the form

// Get data from form
    $body_input=isset($_POST['body_input'])?$_POST['body_input']:'';
    $subject_input=isset($_POST['subject_input'])?$_POST['subject_input']:'';

// Loop sending a message for each recipient
foreach(array_column($user_ids, 'user_id') as $user_N) {
    send_msg($user_N, $body_input, $subject_input);    
}

I want to edit the subject/title and message body before send_msg

I have tried

$args = array( 'recipients' => $user_id, 'sender_id' => bp_loggedin_user_id(), 'subject' => $title.'some text', 'content' => $message );

which does work but when I try

$args = array( 'recipients' => $user_id, 'sender_id' => bp_loggedin_user_id(), 'subject' => $title.'some text', 'content' => $message.'some text' );

It sends the message twice, one with title and body and another with title.what_I_append and body.what_I_append

I have also tried this way but this does not post

$body_input=isset($_POST['body_input'])?$_POST['body_input']:''.'some text to append';

How can I correctly concatenate to the variables used for subject and body?

A quick answer to follow my comments:

I would recommend send_msg() focus's only on sending the message.

Therefore append to the subject and body prior to sending these values to send_msg() function.

<?php

//Personally i wouldnt append to an empty value, your choice.
$subject = isset($_POST['subject_input']) 
    ? $_POST['subject_input'].' appended text'
    : ''
$body= isset($_POST['body_input']) 
    ? $_POST['body_input'].' appended text' 
    : '';

send_msg($user_id, $title, $body);

Hope this helps