To get the underlying file path of a tmpfile file pointer:

<?php
$file = tmpfile();
$path = stream_get_meta_data($file)['uri']; // eg: /tmp/phpFx0513a

I found this function useful when uploading a file through FTP. One of the files I was uploading was input from a textarea on the previous page, so really there was no "file" to upload, this solved the problem nicely:

<?php
    # Upload setup.inc
    $fSetup = tmpfile();
    fwrite($fSetup,$setup);
    fseek($fSetup,0);
    if (!ftp_fput($ftp,"inc/setup.inc",$fSetup,FTP_ASCII)) {
        echo "<br /><i>Setup file NOT inserted</i><br /><br />";
    }
    fclose($fSetup);
?>

The $setup variable is the contents of the textarea.

And I'm not sure if you need the fseek($temp,0); in there either, just leave it unless you know it doesn't effect it.

at least on Windows 10 with php 7.3.7, and Debian Linux with php 7.4.2,

the mode is not (as the documentation states) 'w+' , it is 'w+b'

(an important distinction when working on Windows systems)

Since this function may not be working in some environments, here is a simple workaround:

function temporaryFile($name, $content)
{
    $file = DIRECTORY_SEPARATOR .
            trim(sys_get_temp_dir(), DIRECTORY_SEPARATOR) .
            DIRECTORY_SEPARATOR .
            ltrim($name, DIRECTORY_SEPARATOR);

    file_put_contents($file, $content);

    register_shutdown_function(function() use($file) {
        unlink($file);
    });

    return $file;
}

To get tmpfile contents:
<?php
  $tmpfile = tmpfile();
  $tmpfile_path = stream_get_meta_data($tmpfile)['uri'];
  // ... write to tmpfile ...
  $tmpfile_content = file_get_contents($tmpfile_path);
?>

Perhaps not the best way for production code, but good enough for logging or a quick var_dump() debug run.

No, the fseek() is necessary - after writing to the file, the file pointer (I'll use "file pointer" to refer to the current position in the file, the thing you change with fseek()) is at the end of the file, and reading at the end of the file gives you EOF right away, which manifests itself as an empty upload.

Where you might be getting confused is in some systems' requirement that one seek or flush between reading and writing the same file.  fflush() satisfies that prerequisite, but it doesn't do anything about the file pointer, and in this case the file pointer needs moving.

-- Josh