Plugin could not be activated

[SOLVED]
with me this was because my PHP version was earlier than PHP 5.3
I was running 5.2.10 (run ‘php – version’ on the command line)
Apparently PHP 5.3 code changed significantly. This post offers a workaround:
http://ww.wp.xz.cn/support/topic/older-php-problems
It references code here, so as to replace the code in the plugin file ‘ wp-fail2ban.php’:
http://pastebin.com/qLCtmMwz

In case this pastebin file has disappeared, here it is, without the introductory information from Charles Lecklider:

<?php
function f2b_openlog()
{
        openlog('wordpress('.$_SERVER['HTTP_HOST'].')',
                         LOG_NDELAY|LOG_PID,
                         defined(WP_FAIL2BAN_LOG) ? WP_FAIL2BAN_LOG : LOG_AUTH);
}

function f2b_bail()
{
        ob_end_clean();
        header('HTTP/1.0 403 Forbidden');
        header('Content-Type: text/plain');
        exit('Forbidden');
}

function f2b_remote_addr()
{
        if (defined('WP_FAIL2BAN_PROXIES')) {
                if (array_key_exists('HTTP_X_FORWARDED_FOR',$_SERVER)) {
                        $ip = ip2long($_SERVER['REMOTE_ADDR']);
                        foreach(explode(',',WP_FAIL2BAN_PROXIES) as $proxy) {
                                if (2 == count($cidr = explode('/',$proxy))) {
                                        $net = ip2long($cidr[0]);
                                        $mask = ~ ( (2 ^ (32 - $cidr[1])) - 1 );
                                } else {
                                        $net = ip2long($proxy);
                                        $mask = -1;
                                }
                                if ($net == $ip & $mask) {
                                        return (false===($len = strpos($_SERVER['HTTP_X_FORWARDED_FOR'],',')))
                                                        ? $_SERVER['HTTP_X_FORWARDED_FOR']
                                                        : substr($_SERVER['HTTP_X_FORWARDED_FOR'],0,$len);
                                }
                        }
                }
        }

        return $_SERVER['REMOTE_ADDR'];
}

function f2b_blocked_users($user, $username, $password){
        if (!empty($username) && preg_match('/'.WP_FAIL2BAN_BLOCKED_USERS.'/i', $username)) {
                f2b_openlog();
                syslog(LOG_NOTICE,"Blocked authentication attempt for $username from ".f2b_remote_addr());
                f2b_bail();
        }
        return $user;
}

function f2b_block_user_enumeration($redirect_url, $requested_url){
        if (intval(@$_GET['author'])) {
                f2b_openlog();
                syslog(LOG_NOTICE,'Blocked user enumeration attempt from '.f2b_remote_addr());
                bail();
        }
        return $redirect_url;
}

function f2b_login($user_login, $user){
        f2b_openlog();
        syslog(LOG_INFO,"Accepted password for $user_login from ".f2b_remote_addr());
}

function f2b_login_failed($username){
        f2b_openlog();
        syslog(LOG_NOTICE,"Authentication failure for $username from ".f2b_remote_addr());
}

if (defined('WP_FAIL2BAN_BLOCKED_USERS')) {
        add_action( 'authenticate','f2b_blocked_users',1,3);
}

if (defined('WP_FAIL2BAN_BLOCK_USER_ENUMERATION')) {
        add_filter( 'redirect_canonical','f2b_block_user_enumeration',10,2);
}

add_action( 'wp_login','f2b_login',10,2);
add_action( 'wp_login_failed','f2b_login_failed');