You are looking at a piece of obfuscated code. I will explain it line by line, but first let's go over the functions that are used:
base64_decode()
This function decodes a base64 encoded string. It's used here to unscramble intentionally scrambled code.
gzinflate()
This function decompresses a compressed string. It's used the same way as base64_decode().
eval()
This function executes a string as code. Its use is discouraged and is in itself a bit of a red flag, though it has legitimate uses.
$keystroke1 = base64_decode("d2RyMTU5c3E0YXllejd4Y2duZl90djhubHVrNmpoYmlvMzJtcA==");
This line creates an apparently random string of characters: wdr159sq4ayez7xcgnf_tv8nluk6jhbio32mp
This string is saved to a variable, $keystroke1. The string itself is not important, other than that it contains some letters that are used later.
eval(gzinflate(base64_decode('hY5NCsIwEIWv8ixdZDCKWZcuPUfRdqrBmsBkAkrp3aVIi3Tj9v1+vje7PodWfQwNv3zSZAqJyqGNHRdE4+JiVU2ZVHy42fLyjDkoYUT54DdqpHxNKmsAJwtHFXxvksrAYXGort1cE9YsAe1dTJTOzCuEPZbhChN4SPw/iePMd/7ybSmcxeb+4Mj+vkzTBw==')));
This line unscrambles a doubly scrambled string and then runs this resulting code:
if(!function_exists("rotencode")){function rotencode($string,$amount) { $key = substr($string, 0, 1); if(strlen($string)==1) { return chr(ord($key) + $amount); } else { return chr(ord($key) + $amount) . rotEncode(substr($string, 1, strlen($string)-1), $amount); }}}
This creates a new function called rotencode(), which is yet another way of unscrambling strings.
$O0O0O0O0O0O0=$keystroke1[2].$keystroke1[32].$keystroke1[20].$keystroke1[11].$keystroke1[23].$keystroke1[15].$keystroke1[32].$keystroke1[1].$keystroke1[11];
This line takes specific characters from that random string from earlier to create the word "rotencode" as a string, stored in the variable named $O0O0O0O0O0O0.
$keystroke2 = $O0O0O0O0O0O0("xes26:tr5bzf{8ydhog`uw9omvl7kicjp43nq", -1);
This line uses the rotencode() function to unscramble yet another string (actually exactly the same string as before, for some reason).
$OO000OO000OO=$keystroke2[16].$keystroke2[12].$keystroke2[31].$keystroke2[23].$keystroke2[18].$keystroke2[24].$keystroke2[9].$keystroke2[20].$keystroke2[11];
$O0000000000O=$keystroke1[30].$keystroke1[9].$keystroke1[6].$keystroke1[11].$keystroke1[27].$keystroke1[8].$keystroke1[19].$keystroke1[1].$keystroke1[11].$keystroke1[15].$keystroke1[32].$keystroke1[1].$keystroke1[11];
On these lines the two (identical but separate) random strings are used to create the words gzinflate and base64_decode. This is done so the coder can use these functions without it being apparent that that's what is happening. However, base64_decode() is never used this way in the snippet you posted. That might suggest that it is used later in the code in places you haven't seen or recognized yet. Searching your code for "$O0000000000O" might yield other uses.
eval($OO000OO000OO(base64_decode('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')));
This is where it all comes together. This line unscrambles a line of code which has been compressed and encoded 10 times over. The final result is this:
$cnk = array('localhost');
That's it. It sets the string "localhost" as the sole element of an array and saves it in a variable named $cnk.
In and of itself, there's nothing hazardous about running this code, but noting the lengths that the coder went to in order to hide this line, it's probably a safe bet that it wasn't placed there to help you - the buyer - in any way. Search your code for the $cnk variable if you want to know exactly what's being done. Or better yet, chalk this experience down to a loss and find a better way to learn coding. There are plenty of books, video tutorials and free resources online. Do not place your trust in whoever sold you this code. While they may not have been malicious (people suggested in comments that this might be part of a license check), anyone who includes something like this in their code is not someone you should be learning from.
Good luck on your coding journey!
