How to zip the selected files of a HTML form - php

Question

I have PHP page which lists some songs based on a criteria such as album year or something. I just placed a checkbox near to every song. When a visitor selects some check box and press submit button the form sends each value to a php file. In the PHP file the values are stored in an array.
$files_to_zip=array();
for($i=0;$i<count($_POST['song'];i++)
$files_to_zip[]=$_POST['song'][$i];
//'song' is the name of the checkbox in the HTML form
The problem is the PHP file does not create the zip file with the code i wrote.
function create_zip($files = array(),$destination = '',$overwrite = true) {
//if the zip file already exists and overwrite is false, return false
if(file_exists($destination) && !$overwrite) { return false; }
//vars
$valid_files = array();
//if files were passed in...
if(is_array($files)) {
//cycle through each file
foreach($files as $file) {
//make sure the file exists
if(file_exists($file)) {
$valid_files[] = $file;
}
}
}
//if we have good files...
if(count($valid_files)) {
//create the archive
$zip = new ZipArchive();
if($zip->open($destination,$overwrite ? ZIPARCHIVE::OVERWRITE : ZIPARCHIVE::CREATE) !== true) {
return false;
}
//add the files
foreach($valid_files as $file) {
$zip->addFile($file,$file);
}
//debug
//echo 'The zip archive contains ',$zip->numFiles,' files with a status of ',$zip->status;
//close the zip -- done!
$zip->close();
//check to make sure the file exists
return file_exists($destination);
}
else
{
return false;
}
}
$files_to_zip = array();
foreach($_POST['song'] as $fileabc)
{
$files_to_zip[]=$fileabc;
}
//if true, good; if false, zip creation failed
$result = create_zip($files_to_zip,'my-archive.zip');
if($result==true)echo "Success"; else echo "failed";
This is the entire code I use in the PHP.
If I set the value for $files_to_zip dynamically such as,
$files_to_zip=array('link1','link2','link3'..)
it works properly.
What is the bug in my code?