unclear regarding uploading of csv file in php - php

Question

So I want to:
upload a csv file which will contain a list of student numbers, one to each line (392232, per line).
populate an array with the student numbers (as i already have a process in place of looking up ids from an array of student numbers and storing etc if they were to add students manually)
I have been lookin at a tutorial found here.
however I am slightly confused with this:
if(isset($_FILES['csv_file']) && is_uploaded_file($_FILES['csv_file']['tmp_name'])){...
where does he establish 'tmp_name' from?
anyway, if somebody could explain how I should be going about this I would appreciate the help.
many thanks,
EDIT: added progress of where it is not working.
if(isset($_POST['csv_submit'])){
if(isset($_FILES['csv_file']) && is_uploaded_file($_FILES['csv_file']['tmp_name'])){
//upload directory
$upload_dir = "/ece70141/csv_files/";
//create file name
$file_path = $upload_dir . $_FILES['csv_file']['name'];
//move uploaded file to upload dir
// GETTING THE ERROR BELOW.
if (!move_uploaded_file($_FILES['csv_file']['tmp_name'], $file_path)) {
//error moving upload file
echo "Error moving file upload";
}
print_r($_FILES['csv_file']);
//delete csv file
unlink($file_path);
}
}

Answer 1

$_FILES is a magic superglobal similar to $_POST. It's an array of every file that's been uploaded in the last request, and where that file is stored (tmp_name).
tmp_name is basically generated by the web server to let PHP know where they've stored the file.
You have the following items available to you in each element of the $_FILES array:
name (Original Name of the file)
type (MIME Type of the file, ie. text/plain)
tmp_name (Path to the uploaded files' temporary location)
error (Error occurred when uploading, 0 when no error)
size (Size of the uploaded file, in bytes)
From what I can see in your code, this will work perfectly fine and as discussed in comments, I think the issue lies in your HTML.
The tutorial that you linked to has an incorrect <form ..> tag definition. For file uploads, you're required to set the enctype attribute, below is what it should look like:
<form action="" method="post" enctype="multipart/form-data">