This How To has been removed because it is no longer necessary to implement tax using custom PHP in Gravity Forms v1.6.4+.
If you would like to implement tax you can do so by using the Product Field configured as a Calculation field type. Then configure the formula for the calculation to calculate the appropriate tax.
You would do this by adding up the value of all of the Pricing Fields that exist on your form and then multiply them by the appropriate tax rate.
If tax should only be applied in certain situations, you can then use conditional logic to show or hide that field based on the conditions that must exist in order for the tax to be applied.