Solved ! 
The problem was due to the fact that these PHP ~ Javascript functions are not truly the same:
url_encode() ~ escape()
url_decode() ~ solution()
Solution
As explained here, a possible solution is using Javascript equivalents of the url_encode() / url_decode() functions:
http://www.pointwriter.com/blog/index.php?/archives/3-urlencode-for-JavaScript.html
To find the URLDecode function I followed the link to "The URLEncode and URLDecode Page":
http://www.albionresearch.com/misc/urlencode.php
looked at the source and slightly modified the URLDecode() function so that it accepts an argument instead of retrieving its ciphered text from a form input field.
It works perfectly.
On
server side, the RC4 encoding can be done with one of this two calls:
$ciphered_paypal_form_content = endecrypt($cipherKey,$toCipher,'');
or
$ciphered_paypal_form_content = urlencode(rc4Encrypt($cipherKey,$toCipher));
where the
endecrypt function is the one found in manager/includes/crypt.class.inc.php,
while the
rc4Encrypt function comes with
http://farhadi.ir/works/rc4.
On the
client side, the decryption is done this way:
var deciphered = rc4Decrypt(k,URLDecode(document.getElementById('cfc').value));
Note: if your site is in utf-8, since the rc4Encrypt function does not accept utf-8, you would maybe need to insert additional utf8_encode() on the server side and most probably a call to UTFDecode() on the client side.
UTF8Decode() Javascript function can be found at the following address
http://farhadi.ir/works/utf8.
