ZipArchive: `failed to open stream: Is a directory` - php

Question

I keep getting a failed to open stream: Is a directory error while trying to unzip a file with PHP.
This is usually an issue with the destination path.
However, in my case, the destination is fine: I can unzip different files using the same code, or unzip the same file without PHP.
The ZipArchive::extractTo documentation did not help. Nor did other SO threads on this error message.
FYI, this is a Q&A post.
Here is the source code as requested in a comment. I shared the Dropbox link publicly, seeing as this matter is dependent on the format of the ZIP input (/ path in ZIP file, see answer.)
/** Config **/
$url = 'https://www.dropbox.com/sh/nx5792q9syppaoo/AAAJZBWlGNAd_EkmQaFvEwe0a?dl=1';
/** Where to save **/
$zipped = ABSPATH . 'tmp/updates/dialogues.zip'; /* Keep in mind this is public-facing. */
$outputdirectory = ABSPATH . 'tmp/updates/testing'; /* The trailing slash is optinal. */
/** Download the dialogues **/
echo 'Fetching the dialogues...<br>';
if ( ( $dropboxfile = fopen( $url, 'r' ) ) === FALSE ) { /* == has higher precedent over = */
echo 'FATAL ERROR: could not open Dropbox file.';
die();
} else if ( file_put_contents( $zipped, $dropboxfile ) === FALSE ) {
echo 'FATAL ERROR: could not copy Dropbox file.';
die();
}
echo "Dialogues downloaded as zip OK. (See {$zipped})<br>";
/** Unzip the dialogues **/
print 'Unzipping...<br>';
$zip = new ZipArchive;
$res = $zip->open( $zipped );
if ( $res === TRUE ) {
// Does not work
// $zip->extractTo( ABSPATH . 'tmp/updates/testing' );
// Works
for( $i = 0 ; $i < $zip->numFiles ; $i++ ) {
if ( $zip->getNameIndex( $i ) != '/' && $zip->getNameIndex( $i ) != '__MACOSX/_' ) {
print $zip->getNameIndex( $i ) . '<br>';
$zip->extractTo( $outputdirectory, array($zip->getNameIndex($i)) );
}
}
$zip->close();
print 'Done unzipping.<br>';
} else {
print 'FATAL ERROR: unzipping failed.';
}

Answer 1

As it turned out, I had a / "folder" inside the ZIP archive.
This is confirmed by browsing the ZIP archive through the command-line. e.g., on OS X, using unzip -vl dialogues.zip gave me this:
Archive: dialogues.zip
Length Method Size Ratio Date Time CRC-32 Name
-------- ------ ------- ----- ---- ---- ------ ----
0 Defl:N 2 0% 08-22-15 12:07 00000000 /
2154 Defl:N 1064 51% 06-08-15 15:18 602c6793 dialogue001-en.txt
...
4379 Defl:N 1793 59% 08-21-15 13:56 75e1ef52 dialogue106-en.txt
70 Defl:N 39 44% 08-22-15 12:07 985af458 __MACOSX/_
-------- ------- --- -------
775512 343864 56% 312 files
The file is downloaded from Dropbox and I'd rather be able to work with it as it is.
So I just ended up filtering the list of files contained inside the ZIP archive, before deciding to unzip any given file:
print 'Unzipping...<br>';
$zip = new ZipArchive;
$res = $zip->open( $zipped );
if ( $res === TRUE ) {
//
for( $i = 0 ; $i < $zip->numFiles ; $i++ ) {
if ( $zip->getNameIndex( $i ) != '/' && $zip->getNameIndex( $i ) != '__MACOSX/_' ) {
print $zip->getNameIndex( $i ) . '<br>';
$zip->extractTo( $tempdirectory, array($zip->getNameIndex($i)) );
}
}
$zip->close();
print 'Done unzipping.<br>';
} else {
print 'FATAL ERROR: unzipping failed.';
}
Did not see this documented anywhere and wasted close to an hour on this. I hope this will help someone.
I'm still not clear as to whether the / reference inside of the ZIP file is correct. (Comments welcome.)