Here is a code form a PHP+MySQL book I am reading and I am having trouble understanding this code. This code is about checking a file uploaded into a database. (Please excuse any spelling errors if any, I was typing it in)

Q1: How does it know which case to echo? In the whole code, there is no mention of each case.

Q2: Why do they skip case 5?! Or does it not matter which numbers you use(so I can have case 1, case 18, case 2?)

    if($_FILES['userfile']['error']>0)
{
    echo 'Problem: ';
switch($_FILES['userfile']['error'])
{
    case 1: echo 'File exceeded upload_max_filesize';
            break;
    case 2: echo 'File exceeded max_file_size';
            break;
    case 3: echo 'File only partially uploaded';
            break;
    case 4: echo 'No file uploaded';
            break;
    case 6: echo 'Cannot upload file: no temp directory specified';
            break;
    case 7: echo 'Upload failed: Cannot write to disk';
            break;
}
exit;
}