Parse error: syntax error, unexpected T_STRING, expecting ',' or ';' on line 5

I am looking through a PHP tutorial and want to try out one of the examples myself. but when I input the code I get the above error. what is wrong with the following code:

<?php
$crackers = 5;
while ($crackers > 0)
{
echo "Polly wants a cracker!<br>";
$crackers-;
}
?>