<?php
# Take each 
checked value, and give it a name of $delete
foreach($_POST['delete'] as $delete){
    # Make sure the file actually exists
    if(
file_exists("/location/of/directory/".$delete)){
        # Delete each file
        unlink("/location/of/directory/",$delete);
    }else{
        
continue;
    }
}
# Return to form.html
header("Location: form.html");
?>