What type of encryption technique is used when the following nested functions are called in a PHP script?

eval(gzinflate(str_rot13(base64_decode('FZi3ksTWEV. . .
In the case of the script I am looking at, there are 13260 characters of encrypted code in the parameter string.

Can this be decrypted to make sure that there is nothing malicious in the code?

Does anyone know of a site where I can learn about this technique?

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