This is PHP malware that is uploaded to a server. An attacker then accesses the file in a browser and enters a password. The password also acts as a decryption key and decrypts the encrypted block of text which then executes. Once an attacker enters their password, it is stored in a cookie and they don’t need to enter the password again to access the malicious application.
.. The above request is an attempt to install a plugin in the WordPress CMS through the normal file upload method. What surprised us is that this request had a full set of cookies that indicates that the user or bot doing this was signed in and this probably was an actual web browser.