LDR and/or PWM for energy efficient 'smart' dimmer? Answered

You'd only really PWM DC powered lights. Mains filament lighting would be triac dimmer controlled, and 12V lights, operated from a transformer fed by a triac dimmer.  Dimmer circuits can be electrically noisy, if you don't know what you are doing when you design them.

Systems like this can do funny things when traffic passes or trees move in the wind, or clouds etc. You're best using your finger on the switch.
Although this type of thing can be and is done.

L

.  Start here.
.  If that doesn't work, try this.

i wouldnt know about pulse width modulators, but try microprocessors, like the attiny13, that should be good enough.

they have energy saving settings, so if you know a bit of C++ its easy to make it work.