I believe that you are using variable variable names incorrectly. The reason to use variable variables is when as the programmer you won't know what the variables name is but it will be stored in another variable. Your code should look like this, and this should work.
$varvideolength = "videolength".$howmany;
session_register($varvideolength);
That will register the variable $videolengthX where X is whatever value is in $howmany. Your $videolength = $videolength line makes no sense so I removed it.
