Create an encrypted zip archive with PHP

Note: this answer recommends a cryptographic method that is known insecure, even with good password. Please see link from comments and the Winzip QA on AES. Support for in-php AES zip encryption arrives with php 7.2 (and libzip 1.2.0), which means this answer will soon be outdated too. Until then see this answer for how to call out to 7z instead of the zip command, which supports winzip's AES encryption.

You can use this:

<?php echo system('zip -P pass file.zip file.txt'); ?>

Where pass is the password, and file.txt will be zipped into file.zip. This should work on Windows and Linux, you just need to get a free version of zip for Windows ( http://www.info-zip.org/Zip.html#Win32 )

This kind of security can be broken by brute force attacks, dictionary attacks and etc. But it's not that easy, specially if you chose a long and hard to guess password.

As of php 7.2 (which was released a hours ago), the right way to do this is to use additional functionality included in ZipArchive native php code. (thanks to abraham-tugalov for pointing out that this change was coming)

Now the simple answer looks something like this:

<?php
$zip = new ZipArchive();
if ($zip->open('test.zip', ZipArchive::CREATE) === TRUE) {
    $zip->setPassword('secret_used_as_default_for_all_files'); //set default password

    $zip->addFile('thing1.txt'); //add file
    $zip->setEncryptionName('thing1.txt', ZipArchive::EM_AES_256); //encrypt it

    $zip->addFile('thing2.txt'); //add file
    $zip->setEncryptionName('thing2.txt', ZipArchive::EM_AES_256); //encrypt it

    $zip->close();

    echo "Added thing1 and thing2 with the same password\n";
} else {
    echo "KO\n";
}
?>

But you can also set the encryption method by index and not name, and you can set each password on a per-file basis... as well as specify weaker encryption options, using the newly supported encryption options.

This example exercises these more complex options.

<?php
$zip = new ZipArchive();
if ($zip->open('test.zip', ZipArchive::CREATE) === TRUE) { 
     //being here means that we were able to create the file..

     //setting this means that we do not need to pass in a password to every file, this will be the default
    $zip->addFile('thing3.txt');

    //$zip->setEncryptionName('thing3.txt', ZipArchive::EM_AES_128);
    //$zip->setEncryptionName('thing3.txt', ZipArchive::EM_AES_192);
    //you should just use ZipArchive::EM_AES_256 unless you have super-good reason why not. 
    $zip->setEncryptionName('thing3.txt', ZipArchive::EM_AES_256, 'password_for_thing3');

     $zip->addFile('thing4.txt');
    //or you can also use the index (starting at 0) of the file...
    //which means the following line should do the same thing...
    //but just referencing the text.txt by index instead of name..
    //$zip->setEncryptionIndex(1, ZipArchive::EM_AES_256, 'password_for_thing_4'); //encrypt thing4, using its index instead of its name...

    $zip->close();
    echo "Added thing3 and thing4 with two different passwords\n";
} else {
    echo "KO\n";
}
?>

The underlying support for zip encryption is enabled because libzip 1.2.0 introduced support for encryption. So you will need to have php 7.2 and libzip 7.2 in order to run this code... Hopefully this note will be cruft on this answer "real soon"