A (correct) voltage divider will give you 5V PWM. If the 12V is not regulated it would be a good idea to add an [u]over-voltage protection diode[/u].
If it was me, I'd add a capacitor to the voltage divider output to convert the PWM to filtered-variable DC, then read it with an analog input. I'd start with a [u]time constant[/u] around 1/10th of a second.
...Actually if it was me I'd get the "correct" LED strip so I could skip all of this extra hardware & software.