Get the Whitepaper called PHP Web Backdoor Decode. It covers decoding an obfuscated/encoded web backdoor shell, recovering the original source code, and the encrypted password in order to login to the backdoor shell.
You can download it from the following link: https://packetstormsecurity.com/files/download/154252/phpwebbackdoor-decode.pdf
Source: https://packetstormsecurity.com/files/154252/PHP-Web-Backdoor-Decode.html