Loop checkboxes and store them comma separated in string
<?php
// Loop the swagbags[] array of checkboxes
$checkboxes=$_POST['swagbags'];
$swagbags="";
foreach($checkboxes as $chk)
{
$swagbags .= $chk.","; // append each selected checkbox value, comma separated
}
// THE HTML:
<input type="checkbox" name="swagbags[]" value="Compfit">Compfit: Approximately 1,100 Athletes
<input type="checkbox" name="swagbags[]" value="Samson Games">Samson Games: Approximately 200 Athletes
<input type="checkbox" name="swagbags[]" value="5K">Crush 5K: Approximately 750 Athletes
<input type="checkbox" name="swagbags[]" value="Volunteers">Volunteers: Approximately 200 total