My Drupal was hacked: Remote Code Execution – base64_decode() with ‘PCT4BA6ODSE_’
Your site has indeed been hacked.  Many possibilities of a backdoor exist now.
You can start by upgrading to 7.32 or better and downloading the Hacked! module and scanning your system files for changes.
You can also use one of the following commands to find the exploited files.

grep -Rl PCT4BA6ODSE . | xargs sed -i 's/&lt;[?]php.*PCT4BA6ODSE_.*[?]&gt;/&lt;\?php \/\/ RECOVERED FILE - more info at AllAboutTodd.com \?&gt;/g'
find . -type f -newermt 2014-10-01 ! -newermt 2014-10-31 -name &quot;*.php&quot;

The latter command will find all files that end in .php which were last modified in October. To search instead the files for which the permissions were changed in October, replace -newermt with -newerct.