crypt function why do strings match

Below is a test piece of code I am using to test some crypt values. As
you can see I have hard-coded a crypt string ($SetString) and then
have another input field to input a string ($TypeString). What I find
odd is if I type the string "mypasswordd" (two d's on the end) the
variable $TypeString matches the variable $SetString? Even when I try
"mypassword--d" or something crazy, no matter what it matches, I can
type whatever past the last "d" and the crypted string matches
$SetString.

I am curious why?

<?

if(isset($PassToCrypt)) {
$salt = "copymachine";

//crypt current
$SetString = crypt("mypassword", $salt);
$TypeString = crypt($password, $salt);

echo "Inputted String: $TypeString<br>Set String: $SetString";
}

echo "<form name=\"passwords\" action=\"$PHP_SELF\" method=\"POST\">
<input type=\"hidden\" name=\"PassToCrypt\" value=\"1\">
<input type=\"text\" name=\"password\">
<input type=\"submit\" name=\"submit\" value=\"submit\">
</form>";

?>