I've checked ur code and you seem to lack a lot of ";" at the end of your statements and the reason that the value of $payCheck is not displayed is because:
1) $_REQUEST["payCheck"] is empty
2) Your computation is not really a computation but a string that shows how the computation is done
3) Also when computing, the result is assigned as if it was assigning a value to a variable. it should be $z = $x * $y, not $x * $y = $z.
3) You're trying to printout the value of $paycheck when in fact it should be $payCheck, remember php variables are case sensitive
To solve your problem here's what you need to do:
1) correct your computation statement and assign the result to $paycheck
2) do not represent your computation as a string and don't enclose your strings in "(" and ")"
3) assign the result to a variable and output that variable in your echo statement
Other things to do:
1) remove these lines because the request data you are trying to get are never set therefore they are useless
$pay = $_REQUEST["pay"]
$payCheck = $_REQUEST["payCheck"]
2) The logic for your computation with overtime pay is kind of odd because you only compute for the overtimed hours when in fact it should be added to the wage for the normal hours worked.
Instead of "$z =
$x - 40 * $y * 1.5", it should be "$z = ($x *$y) + (($x - 40) * $y * 1.5)". Where "($x * $y)" computes for the wage for regular hours worked and "(($x - 40) * $y * 1.5)" computes for the wage for overtime hours.
I debugged this locally and this is what I've done to make it work so there shouldn't be anymore problems. Cheers!