PHP: ftp_login fails with "failed to create the SSL context"

I'm trying to upload a file to a remote FTP server (which requires FTPES) using PHP. The script I've written works locally, but on the live server ftp_login() returns false and the following warnings appear in the error log:

PHP Warning:  ftp_login(): failed to create the SSL context [...]
PHP Warning:  ftp_login(): AUTH command ok; starting SSL connection. [...]

I know that the login details are correct (since identical code works locally). I can successfully connect to the FTP server from the live server using curl on the command line.

The server is running PHP 5.3.3 (Zend Server on CentOS). I can see from phpinfo that the PHP configure command includes -with-openssl=/usr/local/openssl-0.9.8o

The code is simply this:

$ftpConnection = ftp_ssl_connect('hostname');
if (!$ftpConnection) {
    echo "Failed to connect to FTP Site\n";
    return false;
}
if (!ftp_login($ftpConnection, 'xxxxx', 'xxxxx')) {
    echo "Failed to login to FTP site\n";
    return false;
}

For reference my local box (where this works fine) is running PHP 5.3.3-1ubuntu9.3.

Can anyone point me in the right direction?