Add Multiple Choice Questions to PHP Web Form - php

Question

I have a standard PHP web form with text fields, dropdowns, and radio buttons. When the form is submitted, the submitted information is e-mailed to me. I want to add a multiple choice question with checkboxes. I can produce the form field, but I can't figure out how to include multiple selections for that one question on the e-mail that gets sent when the form is submitted. Ideally, I would like to have the multiple selections on one line and separated by semi-colons (or commas if that's easier)
I've included what I think are the relevant parts of the code. Apologies if I didn't format this post correctly.
<form action="example.php" method="post" autocomplete="off">
<div class="form-row">
<label for="first">
<span>First Name:</span>
<input type="text" id="first" name="first" autocomplete="off" list="autocompleteOff">
</label>
</div>
<div class="form-row">
<label for="last">
<span>Last Name:</span>
<input type="text" id="last" name="last" autocomplete="off" list="autocompleteOff">
</label>
</div>
<div class="form-row">
<label for="email">
<span>E-mail:</span>
<input type="text" id="email" name="email" autocomplete="off" list="autocompleteOff">
</label>
</div>
<div class="form-row-multiple">
<label for="multiple">
<span>Choose all that apply:</span><br>
<input type="checkbox" name="multiple" id="multiple" value="Option 1"><span>Option 1</span>
<input type="checkbox" name="multiple" id="multiple" value="Option 2"><span>Option 2</span>
<input type="checkbox" name="multiple" id="multiple" value="Option 3"><span>Option 3</span>
<input type="checkbox" name="multiple" id="multiple" value="Option 4"><span>Option 4</span> </label>
</div>
<div class="form-row">
<button type="submit" name="submit" id="submit"><b>Submit</b></button>
</div>
</form>
<?php
$first = remove_headers($_POST['first']);
$last = remove_headers($_POST['last']);
$email = remove_headers($_POST['email']);
$multiple = remove_headers($_POST['multiple']);
$message =
"First: $first\n\n" .
"Last: $last\n\n"
"E-mail: $email\n\n"
"Multiple: $multiple\n\n"

Answer 1

Two things to do here,
For multiple checkboxes use the name attribute as an array and remove the id attribute, as an HTML page should have a unique ID throughout.
<input type="checkbox" name="multiple[]" value="Option 1"><span>Option 1</span>
<input type="checkbox" name="multiple[]" value="Option 2"><span>Option 2</span>
<input type="checkbox" name="multiple[]" value="Option 3"><span>Option 3</span>
<input type="checkbox" name="multiple[]" value="Option 4"><span>Option 4</span>
When the form is submitted, multi-valued checkboxes are received as an array. So, we can make it semicolon-separated using the implode() function.
$multiple = implode(';', $_POST['multiple']);