are you talking about decrypting? encrypting term refers to making a clear text into something not readable like so:
$clear = "whassup";
echo md5($clear); //encoding
you have to know that there's one-way encryption and two-way encryption, and one-way encryption can NOT be decrypted (like md5), the other onces - CAN...
check out http://pl.php.net/manual/en/ref.mcrypt.php
for more about decrypting
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