Multiple file upload codeigniter not working - php

Question

I have a form where multiple files can be uplaoded, along with a name field and a date field. The user can clone the inputs and upload as many certificates as they need. My HTML is below:
HTML:
<input type="file" name="certificate[]" />
<input type="text" name="certificate_name[]" class="right" />
<input type="text" name="expiry_date[]" />
I can upload the files one at a time but as multiple uploads it doesn't work, I get an error
A PHP Error was encountered
Severity: Warning
Message: is_uploaded_file() expects parameter 1 to be string, array given
Filename: libraries/Upload.php
Line Number: 161
PHP:
$uid = 1;
if(isset($_FILES['certificate']))
{
$this->uploadcertificate($uid, $this->input->post('certificate_name'), $this->input->post('expiry_date'));
}
function uploadcertificate($uid, $certificate, $expiry_date)
{
$status = "";
$msg = "";
$file_element_name = 'certificate';
$certificate_name = $certificate;
if ($status != "error")
{
$config['upload_path'] = './certificate_files/';
$config['allowed_types'] = 'pdf|doc|docx|txt|png|gif|jpg|jpeg|';
$config['max_size'] = 1024 * 8;
$config['encrypt_name'] = TRUE;
$this->load->library('upload', $config);
if (!$this->upload->do_upload($file_element_name))
{
$status = 'error';
$msg = $this->upload->display_errors();
}
else
{
$data = $this->upload->data();
$file_id = $this->saveCertificate($uid, $data['raw_name'], $data['file_ext'], $certificate_name, $expiry_date);
}
if($file_id)
{
$status = "success";
$msg = "File successfully uploaded";
}
else
{
//unlink($data['full_path']);
$status = "error";
$msg = "Something went wrong when saving the file, please try again.";
}
}
echo json_encode(array('status' => $status, 'msg' => $msg));
}
function saveCertificate($uid, $file_name, $file_ext, $certificate_name, $expiry_date)
{
for ($ix=0; $ix<count($_FILES['certificate']); $ix++)
{
$insert_certificates = array(
'user_ID' => $uid,
'certificate' => $_POST['certificate_name'][$ix],
'certificate_name' => $_POST['child_dob_additional'][$ix],
'expiry_date' => $_POST['expiry_date'][$ix]
);
$insert = $this->db->insert('certificates', $insert_certificates);
//return $insert; //you cant return here. must let the loop complete.
$insert_certificates_history = array(
'user_ID' => $uid,
'certificate' => $_POST['certificate_name'][$ix],
'certificate_name' => $_POST['child_dob_additional'][$ix],
'expiry_date' => $_POST['expiry_date'][$ix]
);
$insert = $this->db->insert('certificates_history', $insert_certificates_history);
}
}
I'm confused as to exactly where I have went wrong. Can anyone point me in the right direction. Many thanks in advance!