The principle of this encryption algorithm

Ask the php experts to see the principles of this encryption algorithm PHPcodefunctionsys_auth ($ str, $ actionencode, $ key) {$ keymd5 ($ key); $ str $ actionencode? $ Str: base64_decode ($ str); search for a php expert and check the principles of this encryption algorithm.

PHP code

  function sys_auth($str, $action = 'encode', $key = ''){    $key = md5($key);    $str = $action == 'encode' ? $str : base64_decode($str);            $strlen = strlen($str);        $keylen = strlen($key);        $code = '';    for($i = 0; $i < $strlen; $i++){        $k = $i % $keylen;        $code .= $str[$i] ^ $key[$k];    }    $code = $action == "decode" ? $code : base64_encode($code);    return $code;}

I echo the code in every sentence:

PHP code

  Function sys_auth ($ str, $ action = 'encoding', $ key = '') {$ key = md5 ($ key); echo'
$ Key: '. $ key; $ str = $ action = 'encoding '? $ Str: base64_decode ($ str); echo'
$ Str: '. $ str; $ strlen = strlen ($ str); echo' of the encryption method'
$ Str length '. $ strlen; $ keylen = strlen ($ key); echo'
$ Keylen length '. $ keylen; $ code = ''; for ($ I = 0; $ I <$ strlen; $ I ++) {$ k = $ I % $ keylen; echo"
No ". $ I. "The \ $ k value of the next loop is ". $ k; $ code. = $ str [$ I] ^ $ key [$ k]; echo"
The ". $ I. 'Round Robin $ k value is'. $ k; echo"
The ". $ I. 'loop $ str [$ I] value is'. $ str [$ I]; echo"
The value of the ". $ I. 'Round Robin $ key [$ k] is'. $ key [$ k]; echo"
The value of ". $ I." The \ $ code of the next loop is ". $ code ."




";}$ Code = $ action =" decode "? $ Code: base64_encode ($ code); echo"
". $ Code; return $ code;}: [code = PHP] $ key: c81e728d9d4c2f636f067f89cc14862c $ str: 123456 $ str length 6 $ keylen length 32 0th cycles $ k value is 0 0th cycles $ k is 0 0th cycles $ str [$ I] value is 1 0th times loop $ key [$ k] value is c 0th cycles $ code value is R 1st cycles $ k value is 1 1st cycles $ k is 1 1st cycles $ str [$ I] is set to 2 1st cycles. $ key [$ k] is set to 8 1st cycles. $ code is set to R 2nd Cycles. $ k is set 2 2nd Cycles $ k is 2 2nd Cycles $ str [$ I] value is 3 2nd Cycles $ key [$ k] value is the value of $ code for 1 2nd cycles the value of $ k for R 3rd cycles is 3 3rd cycles $ k is 3 3rd cycles $ str [$ I] the value of $ key [$ k] is 4 3rd cycles. the value is the value of $ code in e 3rd cycles. The value of $ k in r q 4th cycles is 4 4th cycles. The value of $ k is 4 4th cycles. $ str [$ I] the value is 5 4th cycles. $ key [$ k] and the value is 7 4th cycles. $ code is the value of r q 5th cycles. $ k is the value of 5 5th cycles. $ k value is 5 5th cycles $ str [$ I] value is 6 5th cycles $ key [$ k] value is 2 5th cycles $ code value is R QUgoCUQIE encrypted: ugoCUQIE

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The problem is: the value of $ str [0] for 0th cycles is 1, and the value of $ key [0] is c.
I directly follow the above $ str [0] ^ $ key [0], (that is, 1 ^ 0), but an error is reported. what is the problem, in addition, $ str [0] ^ $ key [0] does not understand

------ Solution --------------------
Manual, operator